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OF  THE.  , 


SCHOOL  COMPENDIUM 


OF  ; 

NATURAL  AND  EXPERIMEiNTAL 
PHILOSOPHI, 

EMBRACING  THE  ELEMENTARY  PRINCIPLES  OF 

MECHANICS,  HYDROSTATICS,  HYDRAULICS,  PNEUMATICS,  ACOUSTICS,  PYRONO- 
MICS,  OPTICS,  ELECTRICITY,  GALVANISM,  MAGNETISM,  ELECTRO- 
MAGNETISM,  MAGNETO-ELECTRICITY,  AND  ASTRONOMY. 

WITH  A  DESCRIPTION  OF  THE 

STEAM  AND  LOCOMOTIVE  ENGINES. 

BY 

RICHARD  GREEN  PARKER,  A.M. 

FftlNCIPAL  OF  THE  JOHNSON  GRAMMAR  SCHOOL,   BOSTON  *,    AUTHOR   OF  AIDS 
TO   ENGLISH  COMPOSITION,   OUTLINES  OF  GENERAL  HISTORY, 
ETC.,  ETC.,  ETC. 


"  Delectniido  pariterque  monendo, 
Prodesse  qiiam  coiispici." 


TWENTY-FTRST EDITION,  WITH  ADDITIONS  AND  IMPROVEMENTS. 

»  NEW  YORK: 

PUBLISHED  BY  A.  S.  BARNES  &  CO., 

NO.  51  .JOHN-STREET. 

CINCINNATI:  — II.  W.  DERBY  &  CO. 
1848. 


At  a  meeting  of  the  School  Committee  of  the  City  of  Boston,  May  8, 
1838, 

Voted,  That  Parker's  Compendium  of  Natural  and  Experimental  Phi- 
losophy be  adopted  and  used  as  a  text-book  in  our  public  schools. 
Attestf 

S.  F.  McCleary,  Secretary. 


TO  THE 

Hon.  SAMUEL  ATKHSTS  ELIOT, 

MAYOR  OF  THE  CITY  OF  BOSTON,  AND  CHAIRMAN  OF  THE  SCHOOL 
COMMrtTEE. 

Sir, 

The  Public  Schools  of  this  city  are  under  many  obligations  to  you,  for 
the  interest  you  have  taken  in  them,  and  for  your  disinterested  exertions 
for  their  improvement.  This  volume,  designed  to  supply  a  want  which 
they  have  long  felt,  affords  an  opportunity  of  acknowledging  the  obliga- 
tion, which  I  gladly  embrace.  The  gratification  which  I  feel  in  seeing 
you  at  the  head  of  our  municipal  institutions,  I  beg  leave  to  express  in 
borrowed  language : — 

Tibi  ut  gratuler  non  est  in  animo ;  sed  contra,  banc  occasionem,  mihi 
sic  oblatam,  nostram  civitatem  gratulandi,  reniti  non  possum.  Quae  omnia 
solita  tua  benevolentia  ut  accipias  quaeso. 

I  am,  Sir,  very  respectfully. 

Your  obedient  Servant, 

RICHARD  GREEN  PARKER 


Entered,  according  to  Act  of  Congress,  in  the  year  1848, 

By  a.  S.  BARNES  &  Co., 

In  the  Clerk's  Office  of  the  District  Court  for  the  Southern  District  ol 
New  York. 


> 


5  ^c.^ 


REMOTE 

PREFACE. 


The  School  Committee  of  the  city  of  Boston  having  recently  furnished 
the  Grammar  Schools  with  apparatus  for  exemplifying  the  principles  of 
Natural  Philosophy,  the  author  of  this  work,  who,  for  twenty  years,  has  been 
at  the  head  of  one  of  these  large  establishments,  and  has  felt  the  want  of 
an  elementary  treatise  unencumbered  with  extraneous  matter^  has  been 
induced  to  attempt  to  supply  the  deficiency.  If  he  is  not  deceived  in  the 
result  of  his  labors,  the  work  will  commend  itself  to  notice  by  the  follow- 
ing features : 

1.  It  is  adapted  to  the  present  state  of  natural  science ;  embraces  a 
wider  field,  and  contains  a  greater  amount  of  information  on  the  respective 
subjects  of  which  it  treats,  than  any  other  elementary  treatise  of  its  size. 
^  2.  It  contains  engravings  of  the  Boston  School  set  of  philosophical 
apparatus;  a  description  of  the  instruments,  and  an  account  of  many  ex- 
periments which  can  be  performed  by  means  of  the  apparatus. 

3.  It  is  enriched  by  a  representation  and  a  description  of  the  Locomo- 
C  five  and  the  Stationary  Steam  Engines,  in  their  latest  and  most  approved 
^  forms. 

^  4.  Besides  embracing  a  copious  account  of  the  principles  of  Electricity 
^  and  Magnetism,  its  value  is  enhanced  by  the  introduction  of  the  science 
y  of  Pyronomics,  together  with  the  new  sciences  Electro-Magnetism  and 
^  Magneto-Electricity. 

^  5.  It  is  peculiarly  adapted  to  the  convenience  of  study  and  of  recitation, 
by  the  figures  and  diagrams  being  first  placed  side  by  side  with  the  lUus- 

lli  trations,  and  then  repeated  on  separate  leaves  at  the  end  of  the  volume. 
The  number  is  also  given,  where  each  principle  may  be  found,  to  which 
allusion  is  made  throughout  the  volume. 

6.  It  presents  the  most  important  principles  of  science  in  a  larger  type  ; 
while  the  deductions  from  these  principles,  and  the  illustrations,  are  con- 
tained in  a  smaller  letter    Much  useful  and  interesting  matter  is  also 


6 


PREFACE. 


crowded  into  notes  at  the  bottom  of  the  page.  By  this  arrangement,  the 
pupil  can  never  be  at  a  loss  to  distinguish  the  parts  of  a  lesson  which  are 
of  primary  importance  ;  nor  will  he  be  in  danger  of  mistaking  theory  and 
conjecture  for  fact. 

7.  It  contains  a  number  of  original  illustrations,  which  the  author  has 
found  more  intelligible  to  young  students,  than  those  with  which  he  has 
met  elsewhere. 

8.  Nothing  has  been  omitted  which  is  usually  contained  in  an  elemen- 
tary treatise. 

A  work  of  this  kind,  from  its  very  nature,  admits  but  little  originality. 
The  whole  circle  of  the  sciences  consists  of  principles  deduced  from  the 
discoveries  of  different  individuals,  in  different  ages,  thrown  into  common 
stock.  The  whole,  then,  is  common  property,  and  belongs  exclusively  to 
no  one.  The  merit,  therefore,  of  an  elementary  treatise  on  natural  science 
must  rest  solely  on  the  judiciousness  of  its  selections.  In  many  of  the 
works  from  which  extracts  have  been  taken  for  this  volume,  the  author  has 
found  the  same  language  and  expressions  without  the  usual  marks  of  quo- 
tation. Being  at  a  loss,  therefore,  whom  to  credit  for  some  of  the  expres- 
sions which  he  has  borrowed,  he  subjoins  a  list  of  the  works  to  which  he 
is  indebted,  with  this  general  acknowledgment ;  in  the  hope  that  it  may 
be  said  of  him  £is  it  was  once  said  of  the  Mantuan  Bard,  that  "  he  has 
adorned  his  thefts,  and  polished  the  diamonds  which  he  has  stolen." 

It  remains  to  be  stated,  that  the  Questions  at  the  bottom  of  the  page, 
throughout  the  volume,  were  not  written  by  the  author,  but  were  prepared 
by  another  hand. 

R.  G  P 

Boston,  March,  1848. 


ADVERTISEMENT 

TO  THE  SEVENTEENTH  EDITION. 


Ten  years  have  elapsed  since  this  work  first  appeared 
in  permanent  form  from  the  hands  of  the  stereotyper. 
During  this  time,  the  author  has  been  gratified  to  learn 
that  sixteen  editions  have  been  called  for  by  the  pub- 
lic ;  but  this  gratification  has  been  mingled  with  re- 
gret that  he  has  been  unable  from  time  to  time  to  make 
such  improvements  as  he  knew  were  needed,  and  which 
the  progress  of  science,  as  well  as  a  more  extended  ex- 
perience, seemed  imperiously  to  demand.  He  gladly 
avails  himself  of  the  present  opportunity,  afforded  by 
the  new  publishers  into  whose  liands  it  has  fallen,  to 
make  such  improvements  as  in  his  opinion  will  render 
it  more  worthy  of  the  liberal  patronage  it  has  received ; 
for  although  it  is  a  long  time  since  the  author  has  had 
any  pecuniary  interest  in  the  work,  he  hopes  that  it  is 
not  true  that  he  has  had  "  no  further  solicitude." 

The  necessity  of  a  revision  of  the  work  at  this  time 
will  appear  from  the  following  statement.  By  a  vote 
of  the  School  Committee  of  Boston  in  1836,  a  certain 
portion  of  philosophical  apparatus  was  introduced  into 
the  Grammar  Schools,  as  an  experiment.  The  appara- 
tus was  designed  to  unite  economy  with  simplicity,  and 
was  confined  to  the  departments  of  Pneumatics  and 
Electricity.  To  this  apparatus  the  book  was  specially 
adapted,  though  not  wholly  confined.  By  a  recent  vote 
of  the  Committee,  (August,  1847,)  apparatus  of  superior 


8 


ADVERTISEMENT 


construction,  and  embracing  a  much  wider  field,  was 
substituted  for  the  cheap  and  defective  sets  that  were  at 
first  mtroduced.    It  becomes  necessary,  therefore,  in  a 
volume  prepared  with  special  reference  to  the  wants  of 
the  Boston  schools,  to  have  regard  to  the  construction 
and  the  character  of  the  instruments  by  which  the  prin- 
ciples of  physical  science  are  illustrated  in  these  large 
establishments ;  the  author  has  therefore  deemed  it  ex- 
pedient to  make  such  a  revision  of  the  whole  work,  as 
will  not  only  render  it  a  convenient  manual  to  accom- 
pany the  new  apparatus,  but  also  embrace  the  recent 
improvements  and  discoveries  by  which  the  branches  of 
science  of  which  it  treats  have  been  enriched.   A  sched- 
ule of  the  new  apparatus  is  subjoined  ;  and  the  author 
mdulges  the  expectation  that  the  present  edition  of  this 
work  will  be  found  more  worthy  than  its  predecessors 
of  the  favor  which  the  public  have  bestowed  upon  it. 

Lilac  Lodge, 
Dedham,  October,  1847. 


•  LIST  OF  WORKS 

WHICH  HAVE   BEEN  CONSULTED,   OR  FROM  WHICH   EXTRACTS  HAVE  BEEN 
TAKEN,  IN  THE  PREPARATION  Of  THIS  VOLUME. 

Annals  Of  Philosophy;  Aniott's  Elements  of  Physics  ;  Bigelow's  Tech- 
BI 'r  r  !f.^'n''^'''f  '  ^h'^n^bers' Dictionary;  Enfield's,  Olmsted's, 
Blair  s,  Bakewell's,  Draper's,  Grund's,  Jones',  Comstock's,  and  Conver- 
sations on  Natural  Philosophy;  Davis'  Manual  of  Magnetism;  Encyclo- 
piBdia  Americana  ;  Franklin's  Philosophical  Papers ;  Henry's  Chemistry  • 
Kings  Manual  of  Electricity  ;  Lardner  on  the  Steam  Engine  ;  Library  o^ 
Useful  Knowledge ;  Paxton's  Introduction  to  the  Study  of  Inatly  ;  pL. 
bour  on  Locomotive  Engines  on  Railways;  Peschel's  Elements  of  Physics • 
Philips' Astronomy;  Sir  John  Herschel's  Astronomy;  SilUman's  Jo'  na 
of  Science;  Singer's  Electricity;  Scientific  Class  Book;  Scientific  Dia- 
wZi'A^      '''"''T'^'^'^'         Year  Book;  Turner's  Chemistry ; 
r  h      t  Worcester's  and  the  American  School  Geo^aphy- 

Lathrop,  Mclntire,  and  Keith  on  the  Globes.  °  ' 


SCHEDULE  OF  PniLOSOPHICAL  APPARATUS  FOR  THE  BOSTON 
GRAMMAR  SCHOOLS.* 


Adopted  by  the  School  Committee,  August,  1847. 


LAWS  OF  MATTER. 

Apparatus  for  illustrating  Inertia. 

Pair  of  Lead  Hemispheres,  for  Cohesion. 

Pair  of  Glass  Plates,  for  Capillary  Attraction. 

LAWS  OF  MOTION. 

Ivory  Balls  on  Stand  for  Collision. 
Set  of  eight  Illustrations  for  Centre  of  Gravity 
Sliding  Frame  for  Composition  of  Forces. 
Apparatus  for  illustrating  Central  Forces. 

MECHANICS. 

Complete  set  of  Mechanicals,  consisting  of  Pulleys ;  Wheel  and  Axle ; 
Capstan  ;  Screw ;  Inclined  Plane ;  Wedge. 

HYDROSTATICS. 

Bent  Glass  Tube  for  Fluid  Level. 
Mounted  Spirit  Level. 
Hydrometer  and  Jar,  for  Specific  Gravity. 
Scales  and  Weights,  for  Specific  Gravity. 
Hydrostatic  Bellows,  and  Paradox. 

HYDRAULICS. 

Lifting,  or  Common  Water-pump. 

Forcing  Pump ;  illustrating  the  Fire-engine. 

Glass  Syphon-cup  ;  for  illustrating  intermitting  Springs. 

Glass  and  Metal  Syphons. 

PNEUMATICS. 

Patent  Lever  Air-pump  and  Clamp. 

Three  Glass  Bell  Receivers,  adapted  to  the  Apparatus. 

Condensing  and  Exhausting  Syringe. 

Copper  Chamber  for  Condensed  Air  Fountain. 

Revolving  Jet  and  Glass  Barrel. 

Fountain  Glass,  Cock,  and  Jet  for  Vacuum. 

Brass  Magdeburg  Hemispheres. 


*  The  cost  of  this  apparatus  is  about  two  hundred  and  sixty  dollars.  It  was 
made  by  Mr.  Joseph  M.  Wightman,  No.  33  Cornhill,  Boston,  and  in  an  eminent 
degree  unites  beauty  with  durabiiity. 


10 


SCHEDULE  OF  PHILOSOPHICAL  APPARATUS. 


Improved  Weight-lifter,  for  upward  pressure. 

Iron  Weight  of  fifty-six  pounds  and  strap,  )  Weight-lifter. 

Flexible  Tube  and  Connectors,  J 

Brass  Plate  and  Sliding  Rod. 

Bolt  Head  and  Jar. 

Tall  Jar  and  Balloon. 

Hand  and  Bladder  Glasses. 

Wood  Cylinder  and  Plate. 

India-rubber  Bag,  for  expansion  of  ail. 

Guinea  and  Feather  Apparatus. 

Glass  Flask  and  Stop-cock,  for  weighing  aif. 

ELECTRICITY, 

Plate  Electrical  Machine. 

Pith-ball  Electrometer. 

Electrical  Battery  of  four  Jars. 

Electrical  Discharger. 

Image  Plates  and  Figure. 

Insulated  Stool. 

Chime  of  Bells. 

Miser's  Plate,  for  shocks. 

Tissue  Figure,  Ball  and  Point. 

Electrical  Flyer  and  Tellurian.  • 

Electrical  Sportsman,  Jar  and  Birds. 

Mahogany  Thunder-house  and  Pistol. 

Hydrogen  Gas  Generator. 

Chains,  Balls  of  Pith,  and  Amalgam. 

OPTICS. 

Glass  Prism,  and  pair  of  Lenses. 

Dissected  Eyeball,  showing  its  arrangement. 

MAGNETISM. 

Magnetic  Needle  on  Stand. 
Pair  of  Magnetic  Swans. 
Glass  Vase  for  Magnetic  Swans. 
Horseshoe  Magnet. 

ASTRONOMY. 

Improved  School  Orrery. 
Tellurian,  or  Season  Machine. 

ARITHMETIC  AND  GEOMETRY. 

Set  of  thirteen  Geometrical  Figures  of  Solids. 

Box  of  sixty-four  one-inch  Cubes,  for  Cube  Root,  &c. 

AUXILIARIES. 

Tin  Oiler;  Glass  Funnel;  Sulphuric  Acid. 
Set  of  Iron  Weights  for  Hydrostatic  Paradox. 


CONTENTS. 


CHAPTER  L 

Page 

Divisions  of  the  subject   17 

CHAPTER  II. 

Of  Matter...   18 

CHAPTER  III. 

Of  Gravity   25 

CHAPTER  IV. 

Mechanics,  or  the  Laws  of  Motion   30 

CHAPTER  V. 

Hydrostatics  *   73 

CHAPTER  VI. 

Hydraulics   90 

CHAPTER  VII. 

Pneumatics   •   97 

CHAPTER  VIII 
Acoustics  

CHAPTER  IX. 

Pyronomics    ^32 


12  CONTENTS. 

CHAPTER  X.  p^g^ 
Optics  


Electricity  , 


CHAPTER  XI. 

  192 


CHAPTER  XII. 

Galvanism,  or  Voltaic  Electricity   2X6 

CHAPTER  XIII. 
Magnetism  and  Electro-Magnetism    230 

CHAPTER  XIV. 
Astronomy  


INTRODUCTION. 


The  term  Philosopliy  literally  signifies,  the  love  of  wisdom ; 
but,  as  a  general  term  it  is  used  to  denote  an  explanation  of  the 
reason  of  things,  or  an  investigation  of  the  causes  of  all  phe- 
nomena, both  of  mind  and  of  matter. 

When  apphed  to  any  particular  department  of  knowledge, 
the  word  Philosophy  implies  the  collection  of  general  laws  or 
principles,  under  which  the  subordinate  facts  or  phenomena 
relating  to  that  subject  are  comprehended.  Thus,  that  branch 
of  Philosophy  which  treats  of  God,  his  attributes  and  perfec- 
tions, is  called  Theology  ;^  that  which  treats  of  the  material 
world  is  called  Physics,  or  Natural  Philosophy ;  that  which 
treats  of  man  as  a  rational  being,  is  called  Ethics,  or  Moral 
Philosophy ;  and  that  which  treats  of  the  mind  is  called  In- 
tellectual Philosophy,  or  Metaphysics.f 


*  The  word  Theology  is  derived  from  two  Greek  words,  the  former 
of  which  {Qeos)  signifies  God,  and  the  latter  {\oyos)  means  a  discourse  ; 
and  these  two  words  combined  in  the  term  Theology,  Hterally  imply 
a  discourse  about  God.  The  latter  of  these  two  Greek  words  (\oyos 
or  logos)  is  changed  into  logy  to  form  English  compounds,  and  it  enters 
into  the  composition  of  many  scientific  terms.  Thus,  we  have  the  words 
mineraZog-y,  the  science  of  minerals;  meteorology,  the  science  which 
treats  of  meteors ;  ichthyoZo^y,  the  science  of  fishes ;  entomoZo^y,  the 
science  of  insects  ;  lithoZo^y,  of  stones  ;  conchology,  of  shells,  &c. 

t  The  word  Metaphysics  is  composed  of  two  Greek  words,  Meta,  (or 
licra,)  which  signifies  beyond,  and  phusis,  (or  (pvais,)  which  signifies  nature, 
and  in  composition  these  words  imply  something  beyond  nature.  From 


14 


INTRODUCTION. 


All  material  things  are  divided  into  two  great  classes,  called 
organized  and  unorganized  matter.  Organized  matter  is  that 
which  is  endowed  with  organs  adapted  to  the  discharge  of  ap- 
propriate functions,  such  as  the  mouth  and  stomach  of  animals, 
or  the  leaves  of  vegetables.  By  means  of  such  organs  they 
enjoy  life.  Unorganized  matter,  on  the  contrary,  possesses  no 
such  organs,  and  is  consequently  incapable  of  life  and  volun- 
tary action. 

Physical  Science,  or  Physics,  with  its  subdivisions  of  Natural 
History,  (including  Zoology,  Botany,  Mineralogy,  Conchology, 
Entomology,  Ichthyology,  &c.,)  and  Natural  Philosophy,  inclu- 
ding its  own  appropriate  subdivisions,  embraces  the  whole  field 
of  organized  and  unorganized  matter. 

The  term  Natural  Philosophy  is  considered  by  some  authoi  s 
as  embracing  the  whole  extent  of  physical  science,  while  others 
use  it  in  a  more  restricted  sense,  including  only  the  general 
properties  of  unorganized  matter,  the  forces  which  act  upon 
it,  the  laws  which  it  obeys,  the  results  of  those  laws,  and  all 
those  external  changes  which  leave  the  substance  unaffected. 
It  is  in  this  sense  that  the  term  is  employed  in  this  work. 

Chemistry,  on  the  contrary,  is  the  science  which  investigates 
the  composition  of  material  substances,  the  internal  changes 
which  they  undergo,  and  the  new  properties  which  they  ac- 


the  latter  of  these  words,  phusis,  ((pvcis,)  we  obtain  the  term  physics,  which 
in  its  most  extended  sense  implies  the  science  of  natui  e  nud  natural  objects, 
comprehending  the  study  or  knowledge  of  whatever  exists.  The  natural 
division  of  all  things  that  exist  is  into  body  and  mind — things  material  and 
immaterial,  spiritual  and  corporeal.  Physics  relates  to  material  things — 
Metaphysics  to  immaterial.  Man,  as  a  mere  animal,  is  included  in  the 
science  of  Physics  ;  but,  as  a  being  possessed  of  a  soul,  of  intellect,  of 
powers  of  perception,  consciousness,  volition,  reason,  and  judgment,  he  be- 
comes a  subject  of  consideration  in  the  science  of  Metaphysics, 


INTRODUCTION. 


15 


quire  by  such  changes.  The  operations  of  Chemistry  may  be 
described  under  the  heads  of,  Analysis  or  decomposition,  and 
Synthesis  or  combination. 

Natural  Philosophy  may  be  said,  pre-eminently,  to  treat  of 
motion,  while  Chemistry  particularly  relates  to  change  or  al- 
teration. By  the  former  we  become  acquainted  with  the  con- 
dition and  relations  of  bodies  as  they  spontaneously  arise 
without  any  agency  of  our  own.  The  latter  teaches  us  how  to 
alter  the  natural  arrangement  of  elements  to  bring  about  some 
particular  condition  that  we  desire.  To  accomphsh  these  ob- 
jects m  both  of  the  departments  of  science  to  which  we  refer, 
we  make  use  of  appliances  called  philosophical  and  chemical 
apparatus,  the  proper  use  of  which  it  is  the  office  of  Natural 
Philosophy  and  Chemistry  respectively  to  explain.  All  philo- 
sophical knowledge  proceeds  either  from  observation  or  experi- 
ment, or  from  both.  It  is  a  matter  of  observation  that  water, 
by  cold,  is  converted  into  ice ;  but  if,  by  means  of  freezing 
mixtures,  or  evaporation,  we  actually  cause  water  to  freeze, 
we  arrive  at  the  same  knowledge  by  experiment. 

By  repeated  observations,  and  by  calculations  based  on  such 
observations,  we  discover  certain  uniform  modes  in  which  the 
powers  of  nature  act.  These  uniform  modes  of  operation  are 
called  laws  ;— and  these  laws  are  general  or  particular  accord- 
ing to  the  extent  of  the  subjects  which  they  respectively  em- 
brace. Thus,  it  is  a  general  law  that  all  bodies  attract  each 
other  in  proportion  to  the  quantity  of  matter  which  they  con- 
tain. It  is  a  particular  law  of  electricity  that  similar  kinds 
repel,  and  dissimilar  kinds  attract  each  other. 

The  collection,  combination,  and  proper  arrangement  of  such 
general  and  particular  laws,  constitute  what  is  called  Science. 
Thus,  we  have  the  science  of  Chemistry,  the  science  of  Geome- 
try, the  science  of  Natural  Philosophy,  &c. 


16 


INTRODUCTION. 


The  terms,  art  and  science,  have  not  always  been  employed 
with  proper  discrimination.  In  general,  an  art  is  that  which 
depends  on  practice  or  performance,  while  science  is  the  ex- 
amination of  general  laws,  or  of  abstract  and  speculative  prin- 
ciples. The  theory  of  music  is  a  science ;  the  practice  of  it  is 
an  art* 

Science  diflfers  from  art  in  the  same  manner  that  knowl- 
edge differs  from  skill.  An  artist  may  enchant  us  with  his  skill, 
although  he  is  ignorant  of  all  scientific  principles.  A  man  of 
science  may  excite  our  admiration  by  the  extent  of  his  knowl- 
edge, though  he  have  not  the  least  skill  to  perform  any  opera- 
tion of  art.  When  we  speak  of  the  mechanic  arts,  we  mean 
the  practice  of  those  vocations  in  which  tools,  instruments,  and 
machinery  are  employed.  But  the  science  of  mechanics  ex- 
plains the  principles  on  which  tools  and  machines  are  construct- 
ed, and  the  effects  which  they  produce.  Science,  therefore, 
may  be  defined,  a  collection  and  proper  arrangement  of  the 
general  principles  or  leading  truths  relating  to  any  subject; 
and  there  is  this  connection  between  art  and  science,  namely— 
''A  principle  in  science  is  a  rule  of  art." 


NATURAL  PHILOSOPHY. 


CHAPTER  I. 

DIVISIONS  OF  THE  SUBJECT. 

1.  Natural  Philosophy  is  the  science  which  treats 
of  the  powers  and  properties  of  natural  bodies,  their 
mutual  action  on  one  another,  and  the  laws  and  opera- 
tions of  the  material  world. 

2.  Some  of  the  principal  branches  of  Natural  Philos- 
ophy are,  Mechanics,  Pneumatics,  Hydrostatics,  Hy- 
draulics, Acoustics,  Pyronomics,  Optics,  Astronomy, 
Electricity,  Galvanism,^  Magnetism,  Electro-Magnetism, 
and  Magneto-Electricity. 

1.  Mechanics  is  that  branch  of  Natural  Philosophy  which 
relates  to  motion  and  the  moving  powers,  their  nature  and 
laws,  with  their  effects  in  machines,  &c. 

2.  Pneumatics  treats  of  the  nature,  properties,  and  effects 
of  air. 

*  It  may  perhaps  bo  questioned  whether  the  subjects  of  Galvanism, 
Electro-Magnetism,  and  Magneto-Electricity,  do  not  more  properly  fall 
within  the  province  of  Chemistry,  as  they  describe  effects  dependent  on 
chemical  action.  As  this  volume  is  designed  for  the  diffusion  of  useful 
information,  a  strict  adherence  to  rigid  classification  has  not  been  deemed 
so  important,  as  to  exclude  the  notice  of  subjects  so  intimately  connected 
with  one  of  the  most  hiteresting  branches  of  Natural  Philosophy. 


1.  What  is  Natural  Philosophy? 

2.  What  are  the  principal  branches  of  Natural  Philosophy  ?  What  is 
Mechanics?   Of  what  does  Pneumatics  treat? 


NATURAL  PHILOSOPHY. 


3.  Hydrostatics  treats  of  the  nature,  gravity,  and  pressure 
of  fluids. 

4.  Hydraulics  treats  of  the  motion  of  fluids,  particularly  of 
water ;  and  the  construction  of  all  kinds  of  instruments  and 
machines  for  moving  them. 

5.  Acoustics  treats  of  the  nature  and  laws  of  sound. 

6.  Pyronomics  treats  of  heat,  the  laws  by  which  it  is  gov- 
erned, and  the  effects  which  it  produces. 

V.  Optics  treats  of  light,  of  color,  and  of  vision,  or  sight. 

8.  Astronomy  treats  of  the  heavenly  bodies,  such  as  the  sun, 
moon,  stars,  comets,  planets,  &c. 

9.  Electricity  treats  of  thunder  and  lightning,  and  the  causes 
by  which  they  are  produced,  both  naturally  and  artificially. 

10.  Galvanism  is  a  branch  of  Electricity. 

11.  Magnetism  treats  of  the  properties  and  eff'ects  of  the 
magnet,  or  loadstone. 

12.  Electro-Magnetism  treats  of  Magnetism  induced  by 
Electricity. 

13.  Magneto-Electricity  treats  of  Electricity  induced  by 
Magnetism. 


CHAPTER  II. 

OF  MATTER*  AND  ITS  PROPERTIES. 

3.  Matter  is  the  general  name  of  every  thing  that 
occupies  space,  or  has  figure,  form,  or  extension. 

*  The  ancient  philosophers  supposed  that  all  material  substances  were 
composed  of  Fire,  Air,  Earth,  and  Water,  and  these  four  substances  were 
called  the  four  elements,  because  they  were  supposed  to  be  the  simple 
substances  of  which  all  things  were  composed.  Modern  science  has 
proved  that  not  one  of  these  is  a  simple  substance,  but  that  there  are  at 
least  fifty -five  simple  substances,  thirty-two  of  which  are  metallic  and 
twenty-four  non-metallic.  The  consideration  of  those  substances  which 
enter  into  the  composition  of  all  matter,  in  whatever  form,  belongs  to 

Of  what  does  Hydrostatics  treat  ?  Hydraulics  ?  Acoustics  ?  Pyronomics  ? 
Optics?  Astronomy?  Electricity?  Of  what  is  Galvanism  a  branch? 
Of  what  does  Magnetism  treat?  Electro-Magnetism?  Magneto-Elec- 
tricity ? 

3.  What  is  Matter? 


MATTER  AND  ITS  PROPERTIES. 


19 


4.  There  are  seven  essential  properties  belonging  to 
all  matter,  namely:  1.  Impenetrability,  2.  Extension, 
3.  Figure,  4.  Divisibility,  5.  Indestructibility,  6.  Inertia, 
and  7.  Attraction. 

1.  These  are  called  essential  properties,  because  no  particle 
of  matter  can  be  deprived  of  them,  or  exist  without  them. 

2.  There  are  certain  other  properties  existing  in  different 
bodies,  called  accidental  properties,  because  they  do  not  ne- 
cessarily exist  in  the  bodies  themselves,  but  depend  upon  their 
connection  with  other  bodies.  Thus,  color  and  weight  are  ac- 
cidental properties,  because  they  do  not  necessarily  exist  in  the 
bodies  that  possess  them,  but  depend  upon  their  connection 
with  other  things. 

3.  There  are  also  certain  terms  used  to  express  the  state  in 
which  matter  exists,  such  as  Porosity,  Density,  Rarity,  Com- 
pressibihty.  Expansibility,  Mobihty,  Elasticity,  Brittleness,  Mal- 
leability, Ductility,  and  Tenacity. 

5.  Impenetrability  is  the  power  of  occupying  a  cer- 
tain space,  so  that  where  one  body  is,  another  cannot 
be,  without  displacing  it. 

1.  Impenetrability  belongs  to  fluids  as  well  as  sohd  bodies. 
The  reason  why  fluids  appear  less  impenetrable  than  solid 
bodies,  is,  that  the  particles  of  which  they  are  composed  move 

the  science  of  Chemistry.  Bodies  which  consist  of  one  simple  substance 
are  called  homogeneous,  while  those  which  consist  of  two  or  more  sim- 
ple substances  are  called  heterogeneous.  Thus,  water  is  a  heterogeneous 
substance,  being  composed  of  two  simple  or  homogeneous  aeriform  fluids, 
called  Hydrogen  and  Oxygen.  An  aeriform  fluid  is  a  fluid  in  the  form  of 
air.  When  the  particles  of  which  matter  is  composed  is  mentioned,  it  is 
to  be  understood  that  the  smallest  imaginable  portion  is  meant,  not  of  the 
homogeneous  substances  of  which  it  may  be  composed,  but  of  the  matter 
itf  elf,  whether  homogeneous  or  heterogeneous. 


4.  How  many  essential  properties  of  matter  are  there  ?  What  are  they  ? 
Why  are  they  called  essential  properties?  What  other  properties  exist  in 
different  bodies  ?  Why  are  they  called  accidental  properties  ?  Are  color 
and  weight  essential  or  accidental  properties?  Why?  What  terms  are 
used  in  Philosophy  to  express  the  state  in  which  matter  exists  ? 

5.  What  is  meant  by  Impenetrability  ?  Does  impenetrability  belong  to 
fluids  ?  Why  do  fluids  appear  less  impenetrable  than  solid  bodies  ?  What 
is  supposed  to  be  the  form  of  the  particles  of  fluids  ? 


20 


XATL'EAL  PHILOSOPHY. 


easily  among  themselves,  on  account  of  their  shght  degree  of 
cohesion.* 

2.  Put  some  water  into  a  tube  closed  at  one  end ;  and  then 
insert  a  piece  of  wood  that  fits  closely  the  inside  of  the  tube. 
It  will  be  impossible  to  force  the  wood  to  the  bottom  of 
the  tube,  unless  the  water  be  first  removed.  The  same  ex- 
periment may  be  made  with  air  instead  of  water ;  and  proves 
that  water,  air,  and  all  other  fluids  aie  equally  sohd,  or  im- 
penetrable, with  the  hardest  bodies. 

3.  The  impenetrabihty  of  water  was  shown  by  an  experi- 
ment made  at  Florence  many  years  ago.  A  hollow  globe  of 
gold  was  filled  with  water,  and  submitted  to  great  pressure. 
The  water  was  seen  to  exude  through  the  pores  of  the  gold, 

-and  covered  it  with  a  fine  dew. 

4.  When  an  open  vial,  not  inverted,  is  plunged  into  a  basin 
of  water,  the  air  will  iush  out  in  bubbles,  to  make  room  for 
the  water ;  and  if  an  inveited  tumbler  or  goblet  be  immersed 
in  water,  the  water  will  not  lise  far  in  the  tumbler  unless  it  be 
inchned  so  that  the  aii-  can  escape.  These  are  fmther  proofs 
of  the  impenetrabihty  of  air. 

5.  When  a  nail  is  driven  into  wood,  or  any  other  substance, 
it  forces  the  particles  asunder,  and  makes  its  way  between 
them :  and  the  wood  is  not  increased  in  size  by  the  addition 
of  the  nail,  because  wood  is  a  porous  substance,  the  particles 
of  which  may  be  compressed,  and  thus  make  way  for  the 
nail. 

*  It  is  a  well-known  fact,  that  a  certain  quantity  of  salt,  the  particles 
of  which  are  supposed  to  be  smaller  than  those  of  water,  can  be  put  into 
a  vessel  full  of  water,  without  causinof  it  to  overflow ;  and  as  the  particles 
of  which  su^r  is  composed  are  smaller  than  those  of  salt,  a  portion  of 
sugar  may  be  added  after  the  fluid  is  saturated  with  salt.  This  mav  be 
accounted  for  by  supposing  that  the  particles  of  fluids 
are  round,  and  therefore  touch  one  another  only  in  a  ^■ 
few  points.  There  will  be  spaces  between  the  parti- 
cles in  the  same  manner  that  there  are  between  larg-e 
baUs  which  are  piled  on  one  another.  Between  these 
spaces  other  smaller  balls  may  be  placed :  and  these 
smaller  balls,  having  spaces  between  them,  will  ad- 
mit others  still  smaller,  as  may  be  seen  in  Figf.  1. 


"VMiat  follows  from  this  ?  What  figure  illustrates  this  ?  ^\liat  example 
can  you  give  to  prove  the  impenetrability  of  water?  What  of  the  air? 
What  of  soLds  ? 


MATTER  AND  ITS  PROPERTIES. 


21 


0.  Extension  is  but  another  name  for  bulk,  or  size ; 
and  it  is  expressed  by  the  terms  length,  breadth,  width, 
height,  depth,  and  thickness.* 

7.  Figure  is  the  form  or  shape  of  a  body.  Two  cir- 
cles or  two  balls  may  be  of  the  same  shape  or  figure, 
while  they  differ  in  extension.  The  limits  of  extension 
constitute  figure. 

8.  Divisibility  is  susceptibility  of  being  divided.  To 
the  divisibility  of  matter  there  is  no  known  limit. 

1.  A  single  grain  of  gold  may  be  hammered  by  a  gold- 
beater until  it  will  cover  fifty  square  inches ;  each  square  inch 
may  be  divided  into  two  hundred  strips  ;  and  each  strip  into 
two  hundred  parts.  One  of  these  parts  is  only  one  two-mil- 
lionth part  of  a  grain  of  gold,  and  yet  it  may  be  seen  with  the 
naked  eye. 

2.  The  particles  which  escape  from  odoriferous  objects  also 
afi'ord  instances  of  extreme  divisibility. 

9.  By  the  Indestructibility  of  matter  is  meant  that  it 
cannot  be  destroyed.  It  may  be  indefinitely  divided, 
or  altered  in  its  form,  color,  and  accidental  properties, 
but  it  must  still  continue  to  exist  in  some  form  through 
all  its  changes  of  external  appearance. 

1.  When  water  disappears,  either  by  boiling  over  a  fire,  or 

*  Length  is  the  extent  from  end  to  end.  Breadth  or  width  is  the  ex- 
tent from  side  to  side.  Heiglit,  depth,  or  thickness,  is  the  extent  from  the 
top  to  the  bottom.  The  measure  of  a  body  from  the  bottom  to  the  top  is 
called  height ;  from  the  top  to  the  bottom  is  called  depth.  Thus  we  speak 
of  the  depth  of  a  well,  the  height  of  a  house,  &c. 


6.  What  is  meant  by  Extension?  What  terms  are  used  to  express  the 
size  of  a  body?  What  is  length?  Breadth?  Height,  depth,  or  thick- 
ness?   What  is  the  difference  between  height  and  depth? 

7.  What  is  meant  by  Figure  ?  May  bodies  be  of  the  same  shape  or 
figure  and  of  different  dimensions  ?  Give  an  example.  What  constitutes 
figure  ? 

8.  W^hat  is  meant  by  Divisibility  ?  Is  there  any  known  limit  to  the 
divisibility  of  matter  ?  Mention  some  examples  of  the  extreme  divisibility 
of  matter. 

9.  What  is  meant  by  the  Indestmctibility  of  matter?  May  it  be 
changed  in  form  and  in  external  appearance?  Give  examples  of  such 
changes. 


22 


NATURAL  PHILOSOPHY. 


evaporating  by  the  heat  of  the  sun,  or,  in  other  words,  when 
"  it  dries  up/'  it  rises  slowly  in  the  fonii  of  steam  or  vapor. 
This  vapor  ascends  in  the  air  and  constitutes  clouds ;  these 
clouds  again  fall  to  the  earth  in  the  shape  of  rain,  snow,  or 
hail,  and  form  spiings,  fountains,  rivers,  ckc.  The  water  on  or 
in  the  earth,  therefore,  is  constantly  changing  its  shape  or 
situation,  but  no  particle  of  it  is  ever  actually  destroyed. 

2.  The  simple  substances  of  which  fuel  is  composed  are  not 
destroyed  when  the  fuel  is  burnt.  Parts  of  them  arise  in  smoke 
or  vapor,  and  the  remainder  is  reduced  to  ashes.  A  body  in 
burning  undergoes  remarkable  changes  ;  but  the  various  parts 
into  which  it  has  been  separated  by  combustion,  continue  in 
existence,  and  retain  all  the  essential  properties  of  bodies. 

10.  Inertia  is  the  resistance  which  inactive  matter 
makes  to  a  change  of  state,  whether  of  motion  or  rest. 

A  body  at  rest  cannot  put  itself  in  motion,  nor  can 
a  body  in  motion  stop  itself. 

A  body,  when  put  in  motion,  will  continue  to  move  for- 
ever, unless  it  be  stopped.  When  a  stone  or  ball  is  thrown 
from  the  hand,  there  are  two  forces  which  continually  operate 
to  stop  it,  namely,  the  resistance  of  the  air,  and  gravitation: 
all  motion  which  is  caused  by  animal  or 
mechanical  power,  will  be  destroyed  by  Fig.  2. 

the  combined  action  of  these  forces.  But 
could  these  forces  be  suspended,  the 
body  in  motion  would  continue  to  move 
forever. 

Fig.  2  represents  the  simple  apparatus, 
of  Mr.  Wightman,  for  illustrating  iner- 
tia. A  ball  and  a  card  being  placed 
upon  the  pillar,  motion  is  given  to  the 
card  by  means  of  a  spring,  but  the  ball 
remains  on  the  pillar. 

11.  Attraction  is  the  tendency  which  different  bodies 
or  portions  of  matter  have,  to  approach  or  to  adhere  to 
each  other. 


10.  What  is  meaut  by  Inertia?    Can  a  body  at  rest  pat  itself  in  mo- 
tion?   Can  a  body  in  motion  stop  itself?    When  a  stone  or  ball  is  thrown 
from  the  hand,  how  many  forces  continually  operate  to  stop  it?  What 
are  they  ?    How  could  a  body  in  motion  be  made  to  move  forever  ?  Ex 
plain  the  apparatus  for  illustrating  inertia. 


MATTEU  AND  ITd  I'ilOl'ERTIES. 

Every  portion  of  matter  is  attracted  by  every  other 
portion  of  matter,  and  tliis  attraction  mcreases  as  he 
quaVitity  of  matte,'  is  increased,  and  d.mmishes  as  the 
quantity  of  matter  is  dimmished. 

12  There  are  two  kinds  of  attraction  belonging  to 
all  matter,  namely,  the  attraction  of  gravitation  and  the 

attraction  of  cohesion.  •         .    j        r>f  Hif 

The  attraction  of  gravitation  is  the  tendency  ot  dit- 
ferent  bodies  to  approach  each  other. 

The  attraction  of  cohesion  is  that  which  causes  the 
particles  of  a  body  to  cohere  to  each  other. 

By  the  attraction  of  gravity,  a  stone  falls  to  the 
ground.  By  the  attraction  of  cohesion,  the  particles 
which  compose  the  stone  are  held  together. 

1  All  matter  is  composed  of  very  minute  particles  which 
are  connected  together  in  different  bodies  by  different  degrees 

The  mrticles  of  which  bodies  are  composed  absolutely 
touch  one  another  in  few  points  only.  There  are  small  spaces 
caned  pores\  between  the  particles;  and  the  Foportion  of 
these  poves  gives  rise  to  the  terms  density  and  ranty.  A  body 
n  which  the  pores  are  small  and  few  m  number.as  called  a 
dense  body.  When  the  pores  are  large  and  numerous,  the 
body  is  said  to  be  rare.  .  , 

Lemity,  therefore,  iraplies  the  closeness  and  compactness  of  the 

*  Cohesive  attraction  is  illustrated  by  nieans  of  a  pair  of 
ispheres,  which  being  pressed  together,  will  be  found  to  cohere.    The  ex- 
pTrimen;  may  be  made  with  equal  success  with  two  bullets  scraped  smooth 
at  the  points  of  contact. 

t  The  pores  of  bodies  are  generally  filled  with  air. 


11.  What  is  attraction  ?  Is  every  portion  of  matter  attracted  by  every 
other  portion  of  matter  7    How  does  this  attraction  increase  and  dimmish  . 

12  How  many  kinds  of  attraction  are  there  belonging  to  all  matter? 
Wh!;  S  the  attrlction  of  gravitation,  or  gravity  7  What  is  ^^^^^^^^^^ 
of  cohesion,  or  cohesive  attraction  ?  What  causes  a  «  «  ^^'^  ^ 
ground?  By  what  are  the  particles  which  compose  the  stone  held  to 
feZtl  Of  what  is  matter  composed?  Is  the  cohesive  power  which 
mi  tes  them  the  same  in  all  bodies?  How  may  cohesive  at  raction  be  il- 
r^ated?  Do  the  particles  of  matter  in  bodies  absolutely  touch  each 
Other?   What  are  the  spaces  between  them  called  / 


24  ^  ^  NATURAL  PHILOSOPHY. 


particles  of  a  hody^  and  indicates  the  quantity  of  matter  contained 
in  it  under  a  given  hulk. 

Rarity  is  the  reverse  of  density ^  and  implies  extension  of  hulk, 
without  increase  of  quantity  of  matter. 

13.  Compressibility  implies  the  reduction  of  the  limits 
of  extension.  Of  this  all  substances  are  susceptible  if  a 
sufficient  force  be  applied.* 

14.  Expansibility  is  the  reverse  of  compressibility, 
and  implies  the  increase  of  the  limits  of  extension. 

15.  Mobility  implies  susceptibility  of  motion. 

16.  Elasticity  is  the  property  which  causes  a  body  to 
resume  its  shape  after  being  compressed  or  expanded. 

Thus,  when  a  bow  is  bent,  its  elasticity  causes  it  to  resume 
its  shape.  India-rubber  possesses  this  property  in  a  remark- 
able degree,  but  the  gases  in  a  still  greater.  The  elasticity  of 
ivory  is  very  perfect,  that  is  to  say,  it  restores  itself  after  com- 
pression with  a  force  very  nearly  equal  to  that  exerted  in 
compressing  it.  Liquids,  on  the  contrary,  have  scarcely  any 
elasticity. 

17.  Malleability  implies  susceptibility  of  extension  un- 
der the  hammer  or  the  rolling-press.  This  property  be- 
longs to  some  of  the  metals,  as  gold,  silver,  iron,  copper, 
&c.,  but  not  to  all ;  and  it  is  of  vast  importance  to  the 
arts  and  conveniences  of  life.  Gold  is  the  most  malle- 
able of  all  metals. 

18.  Brittleness  is  the  reverse  of  malleability,  and  im- 
plies aptness  to  break  into  irregular  fragments.  This 
property  belongs  chiefly  to  hard  bodies. 

*  Sir  Isaac  Newton  conjectured,  that  if  the  earth  were  so  compressed 
as  to  be  absolutely  without  pores,  its  dimensions  might  not  be  more  than  a 
cubic  inch. 

What  do  you  understand  by  density?    What  by  rarity? 

13.  WTiat  is  compressibility?    Are  all  substances  susceptible  of  it? 

14.  What  is  expansibility  ? 

15.  What  is  mobility  ? 

16.  What  is  elasticity  ?  Wliat  substance  possesses  this  property  in  a 
remarkable  degree  ? 

17.  What  is  malleability?  Does  this  property  belong  to  all  the  metals? 
What  metal  possesses  it  in  the  highest  degree  ? 

18.  What  is  brittleness ?    What  bodies  are  most  brittle? 


OF  GRAVITY. 


25 


19.  Ductility  is  thot  property  which  renders  a  sub- 
stance susceptible  of  being  drawn  into  wire. 

Platina  is  the  most  ductile  of  all  metals.  It  can  be 
drawn  into  wire  scarcely  larger  than  a  spider's  web. 

20  Tenacity  impUes  a  great  degree  of  adhesion  among 
the  particles  of  bodies.  The  tenacity  of  bodies  consti- 
tutes their  strength,  or  their  capability  of  sustaining 
weight.  Iron,  on  account  of  its  fibrous  structure,is  very 
tenacious. 


CHAPTER  III. 

OF  GRAVITY. 

21.  The  term  Gravity,  in  Philosophy,  expresses  the 
reciprocal  attraction  of  separate  portions  of  matter. 

All  matter  possesses  this  attraction,  and  all  bodies 
attract  each  other  with  a  force  proportionate  to  their 
size  and  density,  when  at  a  given  distance  from  each 
other. 

A  body  unsupported  falls  to  the  earth.  This  is  caused  by 
the  superior  attraction  of  the  earth,  arising  from  its  density 
and  size. 

22.  The  attraction  of  gravitation  causes  weight. 

1.  When  we  say  that  a  body  weighs  an  ounce,  a  pound,  or 
a  hundred  pounds,  we  express,  by  these  terms,  the  degree  of 
attraction  by  which  it  is  drawn  towards  the  earth. 

2.  Weight,  therefore,  is  the  measure  of  the  eartKs  attraction. 
As  this  attraction  depends  upon  the  quantity  of  matter  there 
is  in  a  body,  it  follows  that  those  bodies  which  contain  the  most 
matter  will  be  most  strongly  attracted,  and  will  consequently 
be  the  heaviest. 


19.  What  is  ductility?    Which  is  the  most  ductile  of  the  metals? 

20.  What  is  tenacity?    Which  is  the  most  tenacious  of  the  metals? 

21.  What  do  you  understand  by  the  term  gravity  ?  Do  all  bodies  pos- 
sess this  attraction?  To  what  is  its  force  proportional ?  If  a  body  be  un- 
supported, will  it  remain  stationary  ?    Why  will  it  fall  ? 

22.  What  causes  weight?  When  you  say  that  a  body  weighs  an  ounpe 
what  do  you  mean  by  it  ?    What,  therefore,  is  weight? 

2 


26  NATURAL  PHILOSOPHY. 

23  The  force  of  gravity  is  greatest  at  the  surface  of 
the  e'arth,  and  decreases  both  upwards  and  downwards, 
but  in  different  degrees.*  ^  tLp 

It  decreases  above  the  surface  as  the  square  of  t.e 
distance  from  the  centre  increases,  r  rom  tne  surface 
to  the  centre  it  decreases,  simply  as  the  aistance  in- 
creases. That  is.  gravity  at  the  surface  of  the  earth 
(which  is  about  4000  miles  from  che  centre)  is  four  times 
more  powerful  than  it  would  be  at  double  taat  distance, 
or  8000  miles  from  the  centre. 

According  to  the  principles  just  stated,  a  body  which  at 
the  surface  of  the  earth  weighs  a  pound,  at  the  centre  of  the 
earth  will  weigh  nothing.  ...      •  ^  .    r  a 

1000  miles  from  the  centre  it  will  weigh  ^  of  a  pound, 

2000  " 


3000 
4000 
8000 
12000 


^-  of  a  pound, 
I  of  a  pormd, 
1  pound. 


,  d:c.t 


*  The  force  of  crravity  is  absolutely  greatest  at  the  centre  of  the  earth  ; 
but  at  that  point  it^is  exerted  in  all  directions,  and  consequently  a  body  at 
that  pomt  would  remain  stationary,  because  there  is  no  superior  attraction 

for  it  to  obey.  ,         .  ,  ^  „„„ 

+  It  follows  from  what  has  been  stated,  with  regard  to  weight  as  a  con- 
sequence of  attraction,  that  if  there  were  but  one  body  in  the  universe,  i 
would  have  no  weight,  because  there  would  be  nothing  to  attract  it.  But 
cohesive  attraction  would  still  exist,  and  keep  the  particles  which  compose 
the  body  united.   As  the  attraction  between  all  bodies  is  mutual,  it  loilows 
that  when  a  stone  or  any  heavy  body  falls  to  the  earth,  the  earth  will  rise 
to  meet  it.    But  as  the  attraction  is  in  proportion  to  the  quantity  of  matter 
each  contains,  the  stone  will  fall  as  much  farther  than  the  earth  rises,  as 
the  earth  exceeds  the  stone  in  mass.    Now  the  earth  is  one  quatnlliou, 
that  is,  one  thousand  miUion  millions  times  larger  than  the  largest  bod) 
which  has  ever  been  known  to  fall  through  our  ^to°^P*>«';^^/"PP^'"f' 
then,  that  such  a  body  should  fall  through  a  distance  of  1000  feet-the 
earth  would  rise  no  more  than  the  hundred  billionth  part  of  an  inch,  a  dis- 
tance altogether  imperceptible  to  our  senses.    The  principle  of  mutual  at- 
traction  is  not  confined  to  the  earth.    It  extends  to  the  sun,  the  planets, 
comets,  and  stars.    The  earth  attracts  each  of  them,  and  each  of  them 

23.  Where  is  the  force  of  gravity  greatest  ?  How  does  it  change'  How 
does  it  decrease  above  the  surface  of  the  earth  1    How  below  ? 


OF  GRAVITY. 


27 


24.  The  direction  in  which  falhng  bodies  approach 
the  surface  of  the  eartli,  is  called  a  vertical  line.*  Such 
lines  are  everywhere  perpendicular  to  the  surface,  and 
when  prolonged  will  meet  nearly  at  the  centre  of  the 
earth. 

For  this  reason  no  two  lines  suspended 
by  weights,  will  be  parallel  to  each  other.  ^' 
Even  a  pair  of  scales,  hanging  perpendicular 
to  the  earth,  are  not  exactly  parallel,  because 
they  point  to  the  same  spot,  namely,  the 
centre  of  the  earth  ;  but  the  convergency  is 
so  small,  that  their  inclination  is  not  percepti- 
ble to  our  senses.  [See  Fig.  3.]  For  the 
same  reason  no  two  bodies  can  fall  to  the 
earth  in  parallel  lines. 

25.  According  to  the  laws  of  attraction,  all  bodies  at 
an  equal  distance  from  the  earth  will  fall  to  it  in  the 
same  space  of  time,  if  nothing  impede  them.  But  bodies 
of  different  density  fall  with  different  degrees  of  velocity, 
on  account  of  the  resistance  of  the  air ;  and  as  heavy 
bodies  overcome  this  resistance  more  easily  than  light 
ones,  the  former  will  fall  with  the  greater  velocity. 

The  resistance  which  the  air  opposes  to  the  fall 
of  bodies,  is  proportioned  to  their  surface,  not  to  their 
weight. 

attracts  the  earth,  and  these  mutual  attractions  are  so  nicely  balanced  by 
the  power  of  God,  as  to  cause  the  regular  motions  of  all  the  heavenly 
bodies,  the  diversity  of  the  seasons,  the  succession  of  day  and  night,  sum- 
mer and  winter,  and  all  the  grand  operations  which  are  described  in  as- 
tronomy. 

*  A  vertical  line  is  sometimes  called  a  plumh-line,  because  it  is  formed 
by  a  weight  suspended  at  rest  from  a  string.  As  the  weight  thus  employ- 
ed is  usually  of  lead,  the  term  plumb,  from  the  Latin  plumbum,  lead,  is 
applied  to  the  line. 


24.  In  what  direction  will  a  fairing  body  approach  the  surface  of  the 
earth?  Will  the  lines  of  suspension  of  different  bodies  ever  be  parallel? 
Where  will  they  meet,  if  sufficiently  produced  ? 

25.  Will  all  bodies  at  equal  distances  from  the  earth,  fall  to  it  in  the 
same  time?  Why  not?  What  bodies  falls  fastest ?  To  what  is  the  re- 
sistance of  the  air  proportional  ? 


28 


N\TURAL  PHILOSOPHY. 


.  Heavy  bodies  caa  be  made  to  float  in  the  air,  instead  of 
falling  immediately  to  the  ground,  by  making  the  extent  ot 
h^r^surface  counterbalance  their  weight  Thus  gold,  which 
is  one  of  the  heaviest  of  all  substances,  when  spread  out  mto 
thin  leaf,  is  not  attracted  by  gravity  with  su&cient  force  to 
overcome  the  resistance  of  the  air;  it  therefore  floats  in  the 
air,  or  falls  slowly. 

26  All  substances  are  influenced  by  gravity,  in  exact 
proportion  to  their  quantity  of  matter,  and  their  distance 
from  the  central  point  of  attraction. 

1  Even  air  itself,  light  as  it  seems,  is  subject  to  this  attrac- 
tion' The  air*  probably  extends  to  a  height  of  more  than 
forty-five  miles  above  the  surface  of  the  earth.  The  pressure 
of  the  upper  parts  of  the  atmosphere  on  those  beneath  ren- 
ders the  air  near  the  surface  of  the  earth  much  more  dense 
fhan  that  in  the  upper  regions.  This  pressure  is  caused  by  the 
attraction  of  the  earth,  or,  what  is  the  same  fj.-r  J^jf^* 
01  tne  air  above;  and  it  would  cause  the  air  to  fall  like  other 
bodies  completely  to  the  earth,  were  it  not  for  the  elasticity  of 
that  portion  which  is  near  the  surface.  _ 

2  The  air  therefore,  of  which  the  atmosphere  is  composed 
exists  in  a  state  of  compression,  which  causes  it  to  be  heaviest 
near  the  surface  of  the  earth. 

*  We  have  no  means  of  ascertaining  the  exact  height  to  which  the  air 
extends.  Sir  John  Herschel  says,  "  Laying  out  of  consideration  all  n.ce 
■  questions  as  to  the  probable  existence  of  a  definite  limit  to  the  atmosphere,, 
bevond  which  there  is  absolutely  and  rigorously  speaking  no  air,  it  is  clear, 
th^t  for  all  practical  purposes  we  may  speak  of  those  regions  which  are  more 
distant  above  the  earth's  surface  than  the  hundredth  part  of  its  diameter  as 
void  of  air,  and  of  course,  of  clouds,  (which  are  nothing  but  visible  vapors, 
diffused  and  floating  in  the  air,  sustained  by  it,  and  rendering 't  t-b.d  as 
mud  does  water.)  It  seems  probable,  from  many  indications,  that  the  great- 
est height  at  which  visible  clouds  over  exist  does  not  exceed  ten  miles ;  at 
which  height  the  density  of  the  air  is  about  an  eighth  part  of  what  it  is  at 
the  level  of  the  sea."  Although  the  exact  height  to  which  the  atmosphere 
extends  has  never  been  ascertained,  it  ceases  to  reflect  the  sun  s  rays  at  a 

greater  height  than  forty-five  miles^  

26  In  what  proportion  are  all  Substances  influenced  by  gravity  ?  Is  air 
affected  by  it?  How  far  does  the  air  extend  above  the  surface  of  the 
earth  '  What  causes  the  air  to  be  more  dense  at  the  surlace  of  the  earth  ? 
What  causes  this  pressure?  Why  does  not  the  air  fall  to  the  earth  like 
other  bodies  ?  Where  is  the  air  heaviest  ?  What  effect  have  gravity  and 
elasticitv  upon  the  air? 


OF  GRAVITY. 


29 


27.  The  specific  gravity  of  bodies  is  a  term  used  to 
xpress  the  relative  weight  of  equal  bulks  of  different 
Dodies.* 

1.  If  we  take  equal  bulks  of  lead,  wood,  cork,  and  air,  we 
find  the  lead  to  be  the  heaviest,  then  the  wood,  then  the  cork, 
and  lastly  the  air.  Hence  we  say  that  the  specific  gravity  of 
cork  is  greater  than  that  of  air,  the  specific  gravity  of  wood  is 
greater  than  that  of  cork,  and  the  specific  gravity  of  lead 
greater  than  that  of  wood,  &c. 

2.  From  what  has  now  been  said  with  respect  to  the  attrac- 
tion of  gravitation  and  the  specific  gravity  of  bodies,  it  appears 
that  although  the  earth  attracts  all  substances,  yet  this  very 
attraction  causes  some  bodies  to  rise  and  others  to  fall. 

3.  Tliose  bodies  or  substances,  the  specific  gravity  of  which 
is  greater  than  that  of  air,  will  fall,  and  those  whose  specific 
gravity  is  less  than  that  of  air,  will  rise ;  or  rather,  the  air 
being  more  strongly  attracted  will  get  beneath  them,  and,  thus 
displacing  them,  will  cause  them  to  rise.  For  the  same  reason, 
cork  and  other  light  substances  will  not  sink  in  water,  because 
the  specific  gravity  of  water  being  greater,  the  water  is  more 
strongly  attracted,  and  will  be  drawn  down  beneath  them. 
[For  a  table  of  the  specific  gravity  of  bodies,  see  Hydrostatics.] 

4.  The  principle  which  causes  balloons  to  rise,  is  the  same 
which  occasions  the  ascent  of  smoke,  steam,  &c.  The  mate- 
l  ials  of  which  a  balloon  is  made,  are  heavier  than  air,  but  their 
extension  is  greatly  increased,  and  they  are  filled  v/ith  an  elas- 
tic liuid  of  a  different  nature,  specifically  lighter  than  air,  so  that 
on  the  whole,  the  balloon  when  thus  filled  is  much  lighter  than 

*  The  quantity  of  matter  in  a  body  is  estimated,  not  by  its  apparent 
size,  but  by  its  weight.  Some  bodies,  as  cork,  feathers,  &c.,  are  termed 
hght ;  others,  as  lead,  gold,  mercury,  &c.,  are  called  heavy.  The  reason 
of  this  is,  that  the  particles  which  compose  the  former  are  not  closely 
packed  together,  and  therefore  they  occupy  considerable  space  ;  while  in 
the  latter  they  are  joined  more  closely  together,  and  occupy  but  little 
room.  A  pound  of  cork  and  a  pound  of  lead,  therefore,  will  differ  very 
much  in  apparent  size,  while  they  are  both  equally  attracted  by  gravity, 
that  is,  they  weigh  the  same. 


27.  What  is  specific  gravity  ?  Illustrate  this.  Does  the  attraction  of 
the  earth  cause  all  bodies  to  fall?  What  bodies  will  fall?  What  rise? 
How  does  the  air  cause  them  to  rise  ?  Why  do  not  cork  and  other  light 
bodiep  sink  in  wate^?    Explain  the  principle  upon  which  balloons  rise. 


30 


NATURAL  PHILOSOPHY. 


a  portion  of  air  of  the  same  size  or  dimensions,  and  it  will  con- 
sequently rise. 

5.  Gravity,  therefore,  causes  bodies  which  are  lighter  than 
air  to  ascend,  those  which  are  of  equal  weight  with  air  to  re- 
main stationary,  and  those  which  are  heavier  than  air  to  de- 
scend; but  the  rapidity  of  their  descent  is  affected  by  the 
resistance  of  the  air which  resistance  is  proportioned  to  the 
extent  of  the  surface  of  the  falling  body. 


CHAPTER  IV. 

MECHANICS,  OR  THE  LAVV^S  OF  MOTION. 

28.  Mechanics  is  that  branch  of  Natural  Philosophy 
which  relates  to  motion  and  the  moving  powers,  their 
nature  and  laws,  with  their  effects  in  machines,  &c. 

29.  Motion  is  a  continued  change  of  place. 

On  account  of  the  inertia  of  matter,  a  body  cannot  put  it- 
self in  motion,  nor  when  it  is  in  motion  can  it  stop  itself. 

30.  The  power  which  puts  a  body  into  motion  is 
called  ^  force  ;  and  the  power  which  has  a  tendency  to 
stop  or  impede  motion  is  called  resistance, 

31.  The  motion  of  a  body  impelled  by  a  single  force 
is  always  in  a  straight  line,  and  in  the  same  direction  in 
which  the  force  acts. 

32.  The  rapidity  with  which  a  body  moves  is  called 
its  velocity. 

What  effect  has  gravity  on -bodies  Ughter  than  the  air  ?  What  effect 
on  bodies  of  equal  weight  ?  What  effect  on  those  that  are  heavier  ?  What 
affects  the  rapidity  of  their  descent  ?  To  what  is  the  resistance  of  the  air 
proportioned  ? 

28.  What  is  Mechanics? 

29.  What  is  motion?  Why  cannot  a  body  put  itself  in  motion?  Why 
cannot  a  body  stop  itself  when  in  motion  ? 

30.  What  is  force  ?    What  is  resistance  ? 

31.  When  is  the  motion  of  a  body  in  a  straight  hue  ?  In  what  direction 
will  it  move  ? 

32.  What  is  meart  by  velocity? 


MECIIANICS. 


31 


33.  The  velocity  of  a  given  body  is  proportional  to 
the  force  by  which  it  is  put  in  motion.* 

34.  The  velocity  of  a  moving  body  is  determined  by 
the  time  that  it  occupies  in  passing  through  a  given 
space.  The  greater  the  space,  and  the  shorter  the  time, 
the  greater  is  the  velocity. 

Thus,  if  one  body  move  at  the  rate  of  six  miles,  and  another 
twelve  miles  in  the  same  time,  the  velocity  of  the  latter  is 
double  that  of  the  former.f 

35.  The  velocity  of  a  body  is  measured  by  the  space 
over  which  it  moves,  divided  by  the  time  which  it  em- 
ploys in  the  motion. 

Thus,  if  a  body  move  one  hundred  miles  in  twenty  hours, 
the  velocity  is  one  hundred  divided  by  twenty,  that  is,  five 
miles  an  hour. 

*  The  mean  velocity  of  rivers  is  about  four  feet  in  a  second  ;  of  a  very 
rapid  stream,  about  13  feet ;  of  a  moderate  wind,  about  10  feet ;  of  a 
storm,  54  feet ;  of  a  violent  hurricane,  125  feet ;  of  sound,  1142  feet ;  of 
atmospheric  air  rushing  into  a  vacuum,  1280  feet ;  of  a  musket-bal!,  1280 
feet ;  of  a  rifle-ball,  1600  ;  of  a  cannon-ball  of  24  pounds,  2400  feet ;  of  a 
point  at  the  surface  of  the  earth,  under  the  equator,  1500  feet ;  of  the 
earth's  centre  in  its  orbit  round  the  sun,  101,061  feet. 

The  average  rate  of  steamers  between  New  York  and  Albany,  exclu- 
sive of  stoppages,  is  ten  and  a  half  miles  per  hour  ;  of  the  mail  trains  on  the 
great  railroads,  about  25  miles  an  hour  ;  of  the  fastest  sailing  vessel,  15 
feet  in  a  second  ;  of  the  swiftest  race-horse,  42  feet  in  a  second. 

t  Velocity  is  sometimes  called  absolute,  and  sometimes  relative.  Ve- 
locity is  called  absolute  when  the  motion  of  a  body  in  space  is  considered 
without  reference  to  that  of  other  bodies.  When,  for  instance,  a  horse  goes 
a  hundred  miles  in  ten  hours,  his  absolute  velocity  is  ten  miles  an  hour. 
Velocity  is  called  relative  when  it  is  compared  with  that  of  another  body. 
Thus,  if  one  horse  travel  only  fifty  miles  in  ten  hours,  and  another  one 
hundred  in  the  same  time,  the  absolute  velocity  of  the  first  horse  is  five 
miles  an  hour,  and  that  of  the  latter  is  ten  miles  ;  but  their  relative  velocity 
is  five  miles. 

33.  To  what  is  the  velocity  of  a  moving  body  proportional? 

34.  How  is  the  velocity  of  a  moving  body  determined  ?  If  one  body  go 
through  six  miles  in  an  hour,  and  another  twelve,  how  does  the  velocity 
of  the  latter  compare  with  that  of  the  former?  What  is  meant  by  abso- 
lute velocity  ?  Give  an  example.  Wheu  is  the  velocity  of  a  body  termed 
relative?    Give  an  example. 

35.  How  is  the  velocity  of  a  body  measured?    Illustrate  this. 


32 


NATURAL  PHILOSOPHY. 


36.  The  time  employed  by  a  body  in  motion  may  be 
ascertained  by  dividing  the  space  by  the  velocity. 

Thus,  if  the  space  be  one  hundred  miles,  and  the  velocity 
five  miles  in  an  hour,  the  time  will  be  one  hundred  divided  by 
five,  which  is  twenty  hours. 

37.  The  space  also  may  be  ascertained  by  multiply- 
ing the  velocity  by  the  time. 

Thus,  if  the  velocity  be  five  miles  an  hour,  and  the  time 
twenty  hours,  the  space  will  be  twenty  multiplied  by  five, 
which  is  one  hundred  miles. 

38.  There  are  three  terms  applied  to  motion  to  ex- 
press its  kind  ;  namely,  uniform,  accelerated,  and  re- 
tarded motion. 

Uniform  motion  is  that  of  a  body  passing  over  equa, 
spaces  in  equal  times. 

Accelerated  motion  is  that  in  which  the  velocity  con- 
tinually increases  as  the  body  moves. 

Retarded  motion  is  that  in  which  the  velocity  de- 
creases as  the  body  moves. 

39.  Uniform  motion  is  produced  by  a  force  having 
acted  on  a  body,  and  then  ceasing  to  act 

A  ball  struck  by  a  bat,  or  a  stone  thrown  from  the  hand,  is 
in  theory  an  instance  of  uniform '  motion ;  and  if  both  the  at- 
traction of  gravity  and  the  resistance  of  the  air  could  be  en- 
tirely removed,  it  would  proceed  onwards  in  a  straight  fine, 
and  with  a  uniform  motion  forever.  But  as  the  resistance  of 
the  air  and  gravity  tend  to  deflect  it,  it  in  fact  becomes  an  in- 
stance first  of  retarded  and  then  of  accelerated  motion. 

40.  Accelerated  motion  is  produced  by  the  continued 
action  of  one  or  more  forces. 


36.  How  do  you  ascertain  the  time  employed  by  a  body  in  motion?  Il- 
lustrate this. 

37.  How  can  you  ascertain  the  space  ?    IHustrate  this. 

38.  How  many  terms  are  applied  to  motion  to  express  its  kind  ?  What 
are  they?    What  is  uniform  motion ?    Accelerated?  Retarded? 

39.  How  is  uniform  motion  produced?  Why  is  not  a  ball  struck  by  a 
bafc,  or  a  stone  thrown  from  the  hand,  an  instance  of  unifoiTn  motion? 
How  can  it  be  made  an  instance  ? 

40.  How  is  accelerated  motion  produced  ? 


MF.CIIANICS.  ^  33 

1.  Thus,  wlien  a  stone  falls  from  a  height,  the  impulse  which 
it  receives  from  gravity  would  be  suflicient  to  bring  it  to  tlie 
ground  with  a  uniform  veh)city.  But  the  stone  while  falhng 
ac  this  rate  is  still  acted  upon  by  gravity  with  an  additional 
force,  which  contmues  to  impel  it  during  the  whole  time  of  its 
descent. 

2.  In  the  first  second  it  falls  sixteen  feet,  three  times  that  distance 
in  the  next,  five  times  in  the  third,  seven  times  in  the  fourth,  and 
so  Oil,  regularly  increasing  its  velocity  according  to  the  numher 
of  seconds  consumed  in  falling. 

3.  The  height  of  a  building,  or  the  depth  of  a  well,  may  thus 
be  measured  by  observing  the  length  of  time  which  a  stone 
takes  in  falling  from  the  top  to  the  bottom.^ 

41.  Retarded  motion  is  produced  when  a  body  in 
motion  encounters  a  force  operating  in  an  opposite  di- 
rection. 

1.  Thus,  when  a  stone  is  thrown  perpendicularly  upwards, 
the  force  of  gravity  is  continually  operating  in  the  opposite 
direction,  and  attracting  it  downwards  to  the  earth.  The  stone 
moves  upwards  slower  and  slower,  until  the  upward  motion 
ceases,  and  the  body  returns  with  accelerated  motion  to  the 
earth.  It  is  found  that  a  body  thrown  perpendicularly  up- 
wards, takes  the  same  length  of  time  in  ascending  that  it  takes 
in  descending. 

2.  Perpetual  motion  has  never  yet  been  produced  by  art; 
and  the  principles  of  mechanics  seem  to  prove  that  such  a 
motion  is  impossible ;  for  although  in  many  cases  of  bodies 
acting  upon  one  another,  there  is  a  gain  of  absolute  motion, 
yet  the  gain  is  always  equal  in  opposite  directions,  so  that  the 

*  The  spaces  through  which  a  body  falls  hi  equal  successive  portions  of 
time,  increase  as  the  odd  numbers  1,  .3,  5,  7,  &c. ;  that  is,  a  falhng  body 
descends  in  the  2d  second  of  its  fall  through  3  times,  and  in  the  3d  second 
through  5  times  the  space  passed  over  in  the  first  second.  But  the  entire 
spaces  through  which  a  body  will  have  fallen  in  any  given  number  of  sec- 
onds, increase  as  the  squares  of  the  times. 


Give  an  instance  of  accelerated  motion.  How  far  does  a  stone  fall  the 
first  second  of  time?  The  second?  Third?  Fourth?  How  can  you 
measure  the  height  of  a  building,  or  the  depth  of  a  well  ? 

41.  How  is  retarded  motion  produced?  Give  an  example.  How  does 
the  time  of  the  ascent  of  a  body  thrown  perpendicularly  upwards,  compare 
with  that  of  its  descent  ?    Why  cannot  perpetual  motion  b®  produced  ? 

2* 


34 


NATURAL  PHILOSOPHY. 


quantity  of  direct  motion  is  never  increased.  But  nature 
abounds  with  examples  of  perpetual  motion,  as  for  instance, 
the  motion  of  the  heavenly  bodies,  described  in  the  science  of 
astronomy. 

42.  The  momentum  of  a  body  is  its  quantity  of  mo- 
tion, or  the  force  with  which  it  would  strike  agamst 
another  body.  It,  is  measured  by  multiplying  its  weight 
by  its  velocity.* 

Thus,  if  a  body  weighing  six  pounds  move  at  the  rate  of 
two  miles  in  a  second  of  time,  its  momentum  may  be  repre- 
sented by  six  multiplied  by  tAvo,  which  is  twelve.  Hence  a 
small  or  a  hght  body  may  be  made  to  strike  against  another 
body  with  a  greater  force  than  a  heavy  one,  simply  by  giving 
it  sufficient  velocity. 

43.  The  action  of  a  body  is  the  effect  which  it  pro- 
duces upon  other  bodies.  Reaction  is  the  effect  which 
it  receives  from  the  body  on  which  it  acts. 

Thus,  when  a  body  in  motion  strikes  against  another  body, 
it  acts  upon  it,  or  produces  action ;  but  it  also  meets  with  re- 
sistance from  the  body  which  is  struck,  and  this  resistance  is 
the  reaction  of  the  body. 

44.  Action  and  reaction  are  always  equal,  but  in  op- 
posite directions. 

1.  Experiments  to  show  the  mutual  action^  and  reaction  of 
bodies,  are  made  with  both  elastic  and  non-elastic  bodies.  Fig.  4 

*  The  quantity  of  motion  communicated  to  a  body  does  not  affect  the 
duration  of  the  motion.  If  but  httle  motion  be  communicated,  the  body 
will  move  slowly.  If  a  great  degree  be  imparted,  it  will  move  rapidly. 
But  in  both  cases  the  motion  will  continue  until  it  is  destroyed  by  some 
external  force. 


42.  What  is  the  momentum  of  a  body?  How  can  the  momentum  of  a 
body  be  ascertained?  Note.  Does  the  quantity  of  motion  communicated 
to  a  body  affect  the  duration  of  the  motion?  If  but  Uttle  motion  is  com- 
municated, how  will  the  body  move  ?  If  a  great  degree  ?  How  long  will 
the  motion  continue  ?  How  can  a  light  body  be  made  to  have  a  greater 
momentum  than  a  heavy  one  ?    Give  an  instance  of  this. 

43.  What  is  meant  by  action?    Reaction?    Illustrate  this. 
44  How  do  action  and  reaction  compare  ? 


MECHANICS. 


35 


represents  two  ivoiy  balls,  A  and  B,  of  equal        ^.^  ^ 
weio-ht,  ike,  suspended  by  tlircads.    If  the  ball 
A  be  drawn  a  little  on  one  side  and  then  let 


i>-o,  it  will  strike  against  the  other  ball  B,  and 
drive  it  off  to  a  distance  equal  to  that  through 
which  the  first  ball  fell ;  but  the  motion  of  A 
will  be  stopped,  because  when  it  strikes  B  it 
receives  in  return  a  blow  equal  to  that  which  it  gave,  but  in  a 
contrary  direction,  and  its  motion  is  thereby  stopped,  or  rather, 
given  to  B.  Therefore,  when  a  body  strikes  against  another, 
the  quantity  of  motion  communicated  to  the  second  body  is 
lost  hy  the  first ;  but  this  loss  proceeds,  not  from  the  blow 
given  by  the  striking  body,  but  from  the  reaction  of  the  body 
which  it  struck. 

2.  Fio-.  5  represents  six  ivory  balls,  of  equal  weight,  sus- 
pended V  threads.  If  the  ball  A  be  drawn  out  of  the  per- 
pendicular, and  let  fall  against  B,  it  will  communicate  its 
motion  to  B,  and  receive  a  reaction  from  it 
which  will  stop  its  own  motion.  But  the  ball 
B  cannot  move  without  moving  C  ;  it  will  there- 
fore communicate  the  motion  which  it  received 
from  A  to  C,  and  receive  from  C  a  reaction 
which  will  stop  its  motion.  In  like  manner  the 
motion  and  reaction  are  received  by  each  of  the 
balls,  D,  E,  F  ;  but  as  there  is  no  ball  beyond  F 

to  act  upon  it,  F  will  fly  off. 

]Sr.  B.  This  experiment  can  be  accurately  performed  by  those 
bodies  only  which  are  perfectly  elastic. 

3.  Fig.  6  represents  two  balls  of  clay,  (which  are  not 
elastic,)  of  equal  weight,  suspended  by  strings.  If 
the  ball  D  be  raised  and  let  fall  against  E,  only 
part  of  the  motion  of  D  will  be  destroyed  by  it, 
(because  the  bodies  are  non-elastic,)  and  the  two 
balls  will  move  on  together  to  d  and  e,  which  are 
less  distant  from  the  vertical  line  than  the  ball  D 
was  before  it  fell.  Still,  however,  action  and  reac- 
tion are  equal,  for  the  action  on  E  is  only  enough  to  make  ii 
move  through  a  smaller  space,  but  so  much  of  D's  motion  is 
now  also  destroyed.* 

*  Figs.  4  and  5,  as  has  been  explained,  show  the  effect  of  action  and 
reaction  in  elastic  bodies,  and  Fig.  6  shows  the  same  effect  in  non-elastic 

Explain  Fig.  4.    Fig.  5     Fig.  6. 


36 


NATURAL  PHILOSOPHY. 


4.  It  is  -upon  the  principle  of  action  and  reaction,  that  birds 
are  enabled  to  fly.  They  strike  the  air  with  their  wings,  and 
the  reaction  of  the  air  enables  them  to  rise,  fall,  or  remain 
stationary  at  will,  by  increasing  or  diminishing  the  force  of 
the  stroke  of  their  wings. 

5.  It  is  likewise  upon  the  same  principle  of  action  and  re- 
action, that  fishes  swim,  or,  rather,  make  their  way  through 
the  water ;  namely,  by  striking  the  water  with  their  fins.f 

6.  Boats  are  also  propelled  by  oars  on  the  same  principle, 
and  the  oars  are  lifted  out  of  the  water,  after  every  stroke,  so 
as  completely  to  prevent  any  reaction  in  a  backward  direction. 

45.  Motion  may  be  caused  either  by  action  or  reac- 
tion. When  caused  by  action  it  is  called  incident,  and 
when  caused  by  reaction  it  is  called  reflected  motion. J 

bodies.  When  the  elasticity  of  a  body  is  imperfect,  an  intemnediate  effect 
will  be  produced  ;  that  is,  the  ball  which  is  struck  will  rise  higher  than  in 
case  of  non-elastic  bodies,  and  less  so  than  in  that  of  perfectly  elastic 
bodies  ;  and  the  striking  ball  will  be  retarded  more  than  in  the  former  case, 
but  not  stopped  completely,  as  in  the  latter.  They  will,  therefore,  both 
move  onwards  after  the  blow,  but  not  together,  or  to  the  same  distance  ; 
but  in  this,  as  in  the  preceding  cases,  the  whole  quantity  of  motion  de- 
stroyed in  the  striking  ball,  will  be  equal  to  that  produced  in  the  ball  struck. 
Connected  with  '*  the  Boston  school  apparatus"  is  a  stand  with  ivory  balls, 
to  give  a  visible  illustration  of  the  effects  of  collision. 

*  The  muscular  power  of  birds  is  much  greater  in  proportion  to  their 
weight  than  that  of  man.  If  a  man  were  furnished  with  wings  sufficiently 
large  to  enable  him  to  fly,  he  would  not  have  sufficient  strength,  or  mus- 
•     cular  power,  to  put  them  in  motion. 

t  The  power  possessed  by  fishes,  of  sinking  or  rising  in  the  water,  is 
greatly  assisted  "by  a  pecuHar  apparatus  furnished  them  by  nature,  called 
an  air-bladder,  by  the  expansion  or  contraction  of  which  they  rise  or  fall, 
on  the  principle  of  specific  gravity. 

t  The  word  incident  implies  falling  upon,  or  directed  towards.  The 
word  reflected  implies  turned  back.  Incident  motion  is  motion  directed 
towards  any  particular  object,  against  which  a  moving  body  strikes.  Re- 
flected motion  is  that  which  is  caused  by  the  reaction  of  the  body  which 
is  struck.    Thus,  when  a  ball  is  thrown  against  a  surface,  it  rebounds  or 


Upon  what  principle  do  birds  fly?  Explain  hov/.  Upon  what  principle 
do  fishes  swim?  Upon  what  principle  do  boats  move  upon  the  water? 
Explain  how. 

45.  How  may  motion  be  caused?  When  caused  by  action  what  is  it 
called  ?   When  caused  by  reaction  what  is  it  termed  ? 


MECHANICS. 


37 


40.  The  angle*  of  incidence  is  the  angle  fornried  by 
the  line  which  the  incident  body  makes  in  its  passage 
towards  any  object,  with  a  line  per-  ^ 
pendicular  to  the  surface  of  the  ob-  ^ 
ject.  '^'--..^^^ 

Thus,  in  Fig.  1,  the  line  ABC  repre-  p  

sents  a  wall,  and  P  B  a  line  perpendicular  ^^.--^ 
to  its  surface.    0  is  a  ball  moving  in  the  ^ 
direction  of  the  dotted  line,  0  B.   The  an- 
o-le  0  B  P  is  the  ano^le  of  incidence. 

is  turned  back.  This  return  of  the  ball  is  called  reflected  motion.  As  re- 
flected motion  is  caused  by  reaction,  and  reaction  is  caused  by  elasticity, 
it  follows,  that  reflected  motion  is  always  greatest  in  those  bodies  which 
are  most  elastic.  For  this  reason,  a  ball  filled  with  air  rebounds  better 
than  one  stuffed  with  bran  or  wool,  because  its  elasticity  is  greater.  For 
the  same  reason,  balls  made  of  caoutchouc,  or  India-rubber,  will  rebound 
more  than  those  which  are  made  of  most  other  substances. 

*  As  this  book  may  fall  into  the  hands  of  some  who  are  unacquainted 
with  geometrical  figures,  a  few  explanations  are  here  subjoined. 

1.  An  angle  is  the  opening  made  by  two  lines  which  meet  each  other  in  a 
point.  The  size  of  the  angle  depends  upon  the  opening,  and  not  upon  the 
length  of  the  lines. 

2.  A  circle  is  a  perfectly  round  figure,  ev-  Fig.  8. 
ery  part  of  the  outer  edge  of  which,  called  the  ^ 
circumference,  is  equally  distant  from  a  point 
within,  called  the  centre.    (See  Fig.  8.) 

3.  The  straight  lines  drawn  from  the  cen- 
tre to  the  circumference  are  called  radii. 
[The  singular  number  of  this  word,  is  radius.] 
Thus,  in  Fig.  8,  the  lines  CD,  CO,  CR,  and 
CA,  are  radii. 

4.  The  lines  drawn  through  the  centre, 

and  terminating  in  both  ends  at  the  circumference,  are  called  diameters. 
Thus,  in  the  same  figure,  D  A  is  a  diameter  of  the  circle. 

5.  The  circumference  of  all  circles  is  divided  into  360  equal  parts,  called 
degrees.  The  diameter  of  a  circle  divides  the  circumference  into  two 
equal  parts  of  180  degrees  each. 

46.  What  is  the  angle  of  incidence?  (Note.—l.  What  is  an  angle? 
Upon  what  does  the  size  of  an  angle  depend?  2.  W^hat  is  a  circle? 
3.  What  are  radii?  What  lines  in  Fig.  8  are  radii?  4.  What  are  diam- 
eters? In  Fig.  8,  what  line  is  the  diameter?  5.  How  is  the  circumfer- 
ence of  all  circles  divided  ?  Into  how  many  parts  does  the  diameter  of  a 
circle  divide  it  ? 


c 

38 


NATURAL  PHILOSOPHY. 


47.  The  angle  of  reflection  is  the  angle  formed  by 
the  perpendicular  with  the  Une  made  by  the  reflected 
body  as  it  leaves  the  surface  against  which  it  struck. 

Thus,  in  Fig.  7,  the  angle  P  B  K  is  the  angle  of  reflection. 

48.  The  angles  of  incidence  and  reflection  are  always 
equal  to  one  another. 

I.  Thus,  in  Fig.  Y,  the  angle  of  incidence,  0  B  P,  and  the 
angle  of  reflection  P  B  R,  are  equal  to  one  another ;  that  is, 
they  contain  an  equal  number  of  degrees. 

6.  All  angles  are  measured  by  the  number  of  degrees  which  they  con- 
tarn.  Thus,  in  Fig.  8,  the  angle  R  C  A,  as  it  includes  one  quarter  of  the 
circle,  is  an  angle  of  90  degrees,  which  is  a  quarter  of  360.  And  the  an- 
gles R  C  O  and  O  C  D  are  angles  of  45  degrees. 

7.  Angles  of  90  degrees  are  right  angles  ;  angles  of  less  than  90  degrees, 
acute  angles,  and  angles  of  more  than  90  degrees  are  called  obtuse  angles. 
Thus,  in  Fig.  8,  RC  A  is  a  right  angle,  OCR  acute,  and  O  C  A  an  ob- 
tuse angle. 

8.  A  perpendicular  line  is  a  line  which  makes  an  angle  of  90  degrees  on 
each  side  of  any  other  line  or  surface  ;  therefore,  it  will  incline  neither  to 
the  one  side  nor  to  the  other.   Thus,  in  Fig.  8,  R  C  is  perpendicular  to  D  A. 

9.  The  tangent  of  a  circle  is  a  line  which  touches  the  circumference, 
without  cutting  it  when  lengthened  at  either  end.  Thus,  in  Fig.  8,  the 
line  R  T  is  a  tangent. 

10.  A  square  is  a  figure  having  four  equal  sides,  and  four  equal  angles. 
These  will  always  be  right  angles.    (See  Fig.  9.) 

II.  A  parallelogram  is  a  figure  whose  opposite  sides  are  equal  and  par- 
allel.   (See  Figs.  10  and  11.)    A  square  is  also  a  parallelogram. 

12.  A  rectangle  is  a  parallelogram  whose  angles  are  right  angles. 

13.  The  diagonal  of  a  square,  of  a  parallelogram,  or  a  rectangle,  is  a 
line  drawn  through  either  of  them,  and  terminating  at  the  opposite  angles. 
Thus,  in  Figs.  9,  10,  and  11,  the  line  AC  is  the  diagonal  of  the  square, 
parallelogram,  or  rectangle. 

6.  How  are  all  angles  measured  ?  Illustrate  this  by  Fig.  8.  7.  How 
many  degrees  do  right  angles  contam ?  Acute?  Obtuse?  Illustrate  these 
angles  by  Fig.  8.  8.  What  is  a  perpendicular  line  ?  What  line  is  per- 
pendicular in  Fig.  8  ?  9.  What  is  a  tangent?  What  line  is  a  tangent  in 
Fig.  8?  10.  What  is  a  square ?  11.  What  is  a  parallelogram?  12.  A 
rectangle?  13.  What  is  a  diagonal?  What  hues  are  diagonals  in  Figs- 
9,  10,  and  11  ?)    Explain  the  angle  of  incidence  by  Fig.  7. 

47.  What  is  the  angle  of  reflection?    Illustrate  this  by  Fig.  7. 

48.  How  do  the  angles  of  incidence  and  reflection  compare  with  each 
other  ?    Illustrate  this  by  Fig.  7. 


MECHANICS. 


39 


2.  From  what  has  now  been  stated  with  regard  to  the 
angles  of  incidence  and  reflection,  it  follows,  that  when  a  ball 
is  thrown  perpendicularly  against  an  object  which  it  c^mnot 
penetrate,  it  will  return  in  the  same  direction ;  but  if  it  be 
thrown  obliquely,  it  will  return  oblicjuely  on  the  opposite  side 
of  the  perpendicular.  The  more  obhquely  the  ball  is  thrown, 
the  more  obliquely  it  will  rebound.* 


COMPOUND  MOTION. 

49.  Compound  motion  is  caused  by  the  operation  of 
two  or  more  forces  at  the  same  time. 

50.  When  a  body  is  struck  by  two  equal  forces  in 
opposite  directions,  it  will  remain  at  rest. 

,51.  A  body  struck  by  two  forces  in  different  direc- 
tions, will  move  in  a  line  between  them.  This  line  will 
be  the  diagonal  of  a  parallelogram,  having  for  its  sides 
the  lines  through  which  the  body  would  pass,  if  urged 
by  each  of  the  forces  separately. 

1.  Let  Fig.  9  represent  a  ball  struck  by  the  two  equal 
forces,  X  and  Y.  In  this  figure,  the  forces  are  inclined  to 
each  other  at  an  angle  of  90  degrees,  or  j,.^  ^ 
a  right  angle.  Suppose  that  the  force  X 
would  send  it  from  C  to  B,  and  the  force 
Y,  from  C  to  D.  As  it  cannot  obey  both, 
it  will  go  between  them  to  A,  and  the  line 
C  A,  through  which  it  passes,  represents 
the  diagonal  of  the  square,  A  B  C  D. 
The  time  occupied  in  its  passage  from  C 
*  It  is  from  a  knowledge  of  these  facts  that  skill  is  acquired  in  many 
different  sorts  of  games,  as  Billiards,  Bagatelle,  &c.  A  ball  may  also,  on 
the  same  principle,  be  thrown  from  a  gun  against  a  fortification,  so  as  to 
reach  an  object  out  of  the  range  of  a  direct  shot. 


What  follows  from  what  has  been  stated  with  regard  to  the  angles  of 
incidence  and  reflection? 

49.  What  is  compound  motion  ? 

50.  In  what  direction  will  a  body,  struck  by  two  equal  forces  in  oppo- 
site directions,  move  ? 

51.  When  struck  by  two  forces  inclined  to  each  other, how  will  it  move? 
What  is  this  fine  called  ?  Illustrate  these,  first,  by  Fig.  9,  which  represents 
a  ball  struck  by  two  equal  forces  in  different  directions. 


40 


NATURAL  PHILOSOPHY. 


to  A  will  be  the  same  as  the  force  X  would  require  to  send  it 
to  B,  or  the  force  Y  to  send  it  to  D. 

2.  If  the  two  forces  acting  on  a  body  are 
unequal,  but  still  operate  at  right  angles  to 
each  other,  the  body  will  move  from  C  to  A 
as  represented  in  Fig.  10  ;  in  which  it  is  to 
be  observed  that  the  force  Y  is  as  much 
greater  than  the  force  X,  as  the  length  of 
the  side  C  D  of  the  rectangle  A  B  C  D,  ex- 
ceeds the  length  of  the  side  C  B. 

3.  When  two  forces  operate  in  the  direc- 
tion of  an  acute  angle,  (see  Fig.  11,)  the 
body  will  move  as  represented  by  C  A,  in 
the  parallelogram  A  B  C  D. 

4.  If  the  forces  operate  in  the  direction  of  an  obtuse  angle, 
the  body  will  move  as  represented  by  D  B  in  the  same  figure. 

52.  Circular  motion  is  motion  around  a  central  point, 
and  is  caused  by  two  forces  operating  at  the  same  time, 
by  one  of  which  it  is  projected  forward  in  a  straight 
line,  while  by  the  other  it  is  deflected  towards  a  fixed 
point. 

The  whirling  of  a  ball,  fastened  to  a  string  held  by  the  hand, 
is  an  instance  of  circular  motion.  The  ball  is  urged  by  two 
forces,  of  which  one  is  the  force  of  projection,  and  the  other  the 
string  which  confines  it  to  the  hand.  The  two  forces  act  at 
right  angles  to  each  other,  and  (according  to  No.  51)  the  ball 
will  move  in  the  diagonal  of  a  parallelogram.  But,  as  the  force 
which  confines  it  to  the  hand  only  keeps  it  within  a  certain 
distance,  without  drawing  it  nearer  to  the  hand,  the  motion  of 
the  ball  will  be  through  the  diagonals  of  an  indefinite  number 
of  minute  parallelograms,  formed  by  every  part  of  the  circum- 
ference of  the  circle. 

53.  There  are  three  different  centres  which  require 
to  be  distinctly  noticed;  namely, -the  centre  of  magni- 
tude, the  centre  of  gravity,  and  the  centre  of  motion. 

Second,  by  Fig.  10,  which  represents  a  ball  struck  by  two  unequal 
forces,  acting  at  right  angles.  Third,  by  Fig.  11,  where  the  forces  operate 
in  the  direction  of  an  acute  angle.  Fourth,  by  Fig.  11,  where  the  forces 
oi>erate  in  the  direction  of  an  obtuse  angle. 

52  What  is  circular  motion?    How  is  it  caused?    Illustrate  th  s. 

53.  How  many  different  centres  are  there  which  require  to  be  noticed? 
Define  each  of  them. 


MECHANICS. 


41 


The  centre  of  magnitude  is  the  central  point  of  the 
bulk  of  a  body. 

The  centre  of  gravity  is  the  point  about  which  all 
the  parts  balance  each  other. 

The  centre  of  motion  is  the  point  around  which  all 
the  parts  of  a  body  move. 

When  the  body  is  not  of  a  size  nor  shape  to  allow 
every  point  to  revolve  in  the  same  plane,  the  line  around 
which  it  revolves  is  called  the  axis  of  motion.* 

54.  The  centre  or  the  axis  of  motion  is  generally  sup- 
posed to  be  at  rest. 

Thus  the  axis  of  a  spinning  top  is  stationary,  while  every 
other  part  is  in  motion  around  it.  The  axis  of  motion  and  the 
centre  of  motion  are  terms  which  relate  only  to  circular  motion. 

55.  The  two  forces  by  which  circular  motion  is  pro- 
duced, are  called  central  forces.  Their  names  are  the 
centripetal  force  and  the  centrifugal  force. f 

56.  The  centripetal  force  is  that  which  confines  a 
body  to  the  centre  around  which  it  revolves. 

The  centrifugal  force  is  that  which  impels  the  body 
to  fly  off  from  the  centre. 

57.  If  the  centrifugal  force  of  a  revolving  body  be 
destroyed,  the  body  will  immediately  approach  the  cen- 
tre which  attracts  it ;  but  if  the  centripetal  force  be 

*  Circles  may  have  a  centre  of  motion  ;  spheres  or  globes  have  an  axis 
of  motion.  Bodies  that  have  only  length  and  breadth  may  revolve  around 
their  own  centre,  or  around  axes  ;  those  that  have  the  three  dimensions  of 
length,  breadth,  and  thickness,  must  revolve  around  axes. 

t  The  word  centripetal  means  seeking  the  centre,  and  centrifugal 
means  flying  from  the  centre.  In  circular  motion,  these  two  forces  con- 
stantly balance  each  other;  otherwise  the  revolving  body  will'either  ap- 
proach the  centre  or  recede  from  it,  according  as  the  centripetal  or  centrif- 
ugal force  is  the  stronger. 

54.  Is  the  centre  or  axis  of  motion  supposed  to  be  at  rest,  or  does  it 
move?    To  what  do  the  terms  centre  of  motion  and  axis  of  motion  relate  ? 

55.  What  are  the  two  forces  called  which  produce  circular  motion? 
What  is  the  name  of  each?  What  do  the  words  centripetal  and  centrifu- 
gal mean? 

56.  Define  a  centripetal  force.    Also  a  centrifugal  force. 

57.  If  the  centrifugal  force  be  destroyed,  to  what  point  will  the  body  tend? 


42 


NATURAL  PHILOSOPHY. 


destroyed,  the  body  will  fly  off'  in  the  direction  of  a  tan- 
gent to  the  curve  which  it  describes  in  its  motion. 

Thus,  when  a  mop  filled  with  water  is  tui-ned  swiftly  round 
bv  the  handle,  the  threads  which  compose  the  head  will  fiy 
off  from  the  centre ;  but  bemg  confined  to  it  at  one  end,  they 
cannot  part  from  it ;  while  the  water  they  contain,  being  un- 
confined,  is  thrown  off  in  straight  lines. 

5S.  The  parts  of  a  body  which  are  farthest  from  the 
centre  of  motion,  move  with  the  greatest  velocity  ;  and 
the  velocity  of  all  the  parts  diminishes,  as  their  distance 
from  the  axis  of  motion  diminishes. 

Fig.  12  represents  the  vanes  of  a  windmill.     The  circles 
denote  the  paths  in  which  the  different  parts  of  the  vanes 
move.    M  is  the  centre  or  axis  of  mo- 
tion around  which  all  the  parts  revolve. 
The  outer  part  revolves  in  the  circle 
D  E  F  G,  another  part  revolves  in  the 
circle  H  I  J  K,  and  the  iimer  part  in 
the  circle  L  X  O  P.     Consequently,  as 
they  all  revolve  around  M  in  the  same 
time,  the  velocity  of  the  parts  which  re- 
volve in  the  outer  cuxle  is  as  much  great- 
er than  the  velocity  of  the  parts  which 
revolve  in  the  mner  circle,  L  X  0  P,  as  the  diameter  of  the 
outer  circle  is  greater  than  the  diameter  of  the  inner. 

59.  As  the  earth  revolves  round  its  axis,  it  follows, 
from  the  preceding  illustration,  that  the  portions  of  the 
earth  which  move  most  rapidly  are  nearest  to  the  equa- 
tor, and  that  the  nearer  any  portion  of  the  earth  is  to 
the  poles,  the  slower  will  be  its  motion. 

60.  Curvilinear  motion  requires  the  action  of  two 
forces  ;  for,  the  impulse  of  one  single  force  always  pro- 
duces motion  in  a  straight  line. 

What  would  be  its  direction  if  the  centripetal  force  were  destroyed? 
Give  an  example. 

58.  What  parts  of  a  body  move  with  the  2n"eatest  velocity  ?  In  what 
proportion  does  the  velocity  of  all  the  parts  diminish?  What  does  Fig.  1'2 
represent  ? 

59.  What  follows,  with  regard  to  the  motion  of  the  earth,  from  the  il- 
lustration of  Fig.  12  ? 

60.  Of  what  is  curvilinear  motion  always  the  result  ?  Why? 


I 


MECHANICS. 


43 


Gl.  A  ball  thrown  in  a  horizontal  direction  is  in- 
fluenced by  three  forces;  namely,  first,  the  force  ol 
projection,  (which  ,G:ives  it  a  horizontal  direct,  n ;) 
second,  the  resistance  of  the  air  through  which  it  passe.-^, 
which  diminishes  its  velocity,  without  changing  its  di- 
rection ;  and  third,  the  force  of  gravity,  which  finally 
brings  it  to  the  ground. 

62.  The  force  of  gravity  is  neither  increased  nor  di- 
minished by  the  force  of  projection.* 

Fio\  13  represents  a  cannon,  loaded  with  a  ball,  and  placed 
on  the  top  of  a  tower,  at  such  a  height  as  to  require  just  three 
seconds  for  another  ball  to  descend  ^. 
perpendicularly.    Now  suppose  the  ^ 


tower  at  the  same  instant.    In  this  « 
figure  C  a  represents  the  perpendicular  line  of  the  falling  ball. 
Qb  is  the  curvilinear  path  of  the  projected  ball,  3  the  ho.izontal 
hne  at  the  base  of  the  tower.    During  the  first  second  of  time, 
the  falling  ball  reaches  1,  the  next  second  2,  and  at  the  end 

*  The  action  of  gravity  being  always  the  same,  the  shape  of  the  curve 
of  every  projectile  (see  No.  63)  depends  on  the  velocity  of  its  motion  ;  but, 
whatever  this  velocity  be,  the  moving  body,  if  thrown  horizontally  from 
the  same  elevation,  will  reach  the  ground  at  the  same  instant.  Thus,  a 
ball  from  a  cannon,  with  a  charge  sufficient  to  throw  it  half  a  mile,  will 
reach  the  ground  at  the  same  instant  of  time  that  it  would  had  the  charge 
been  sufficient  to  throw  it  one,  two,  or  six  miles,  from  the  same  elevation. 
The  distance  to  which  a  ball  will  be  projected,  will  depend  entirely  on  the 
force  with  which  it  is  thrown,  or  on  the  velocity  of  its  motion.  If  it  moves 
slowly,  the  distance  will  be  short— if  more  rapidly,  the  space  pa^ed  over 
in  the  same  time  will  be  greater  ;  but  in  both  cases  the  descent  of  the 
ball  towards  the  earth,  in  the  same  time,  will  be  the  same  number  of  feet, 
whether  it  moves  fast  or  slow,  or  even  whether  it  move  forward  at  all,  or 
not. 


61.  How  many  forces  act  upon  a  ball  thrown  in  a  horizontal  direction? 
What  are  they?    Why  do  bodies  fall  to  the  ground? 

62.  Does  the  force  of  gravity  either  increase  or  decrease  the  fore©  of 
projection  ?    Give  an  illustration. 


cannon  to  be  fired  in  a  horizontal 
direction,  and  at  the  same  instant 
the  other  ball  to  be  dropped  towards 
the  ground.  They  will  both  reach 
the  horizontal  line  at  thse  base  of  the 


44 


NATURAL  PHILOSOPHY. 


of  the  third  second  it  strikes  the  ground.  Meantime,  that  pro- 
jected from  the  cannon,  moves  forward  with  such  velocity,  as 
to  reach  4  at  the  same  time  that  the  falling  ball  reaches  1. 
But  the  projected  ball  falls  downward  exactly  as  fast  as  the 
other,  since  it  meets  the  line  1  4,  which  is  parallel  to  the  hori- 
zon, at  the  same  instant.  During  the  next  second  the  ball 
from  the  cannon  reaches  5,  while  the  other  falls  to  2,  both 
having  an  equal  descent.  During  the  third  second  the  project- 
ed ball  will  have  spent  nearly  its  whole  force,  and  therefore  its 
downward  motion  will  be  greater  while  the  motion  forward  will 
be  less  than  before.  Hence  it  appears  thai  the  horizontal  motion 
does  not  interfere  with  the  action  of  gravity,  hut  that  a  projectile 
descends  with  the  same  rapidity  while  moving  forward  that  it 
would  if  it  were  acted  on  by  gravity  alone.  This  is  the  neces- 
sary result  of  the  action  of  two  forces. 

63.  A  projectile  is  a  body  thrown  into  the  air,  as  a 
rocket,  a  ball  from  a  gun,  or  a  stone  from  the  hand. 

The  force  of  gravity  and  the  liesistance  of  the  air 
cause  projectiles  to  form  a  curve  both  in  their  ascent 
and  descent ;  and  in  descending,  their  motion  is  grad- 
ually changed  from  an  oblique  towards  a  perpendicular 
direction. 

In  Fig.  14  the  force  of  projection  would  carry  a  ball  from  A 
to  D,  while  gravity  would  bring  it  to  C.  If 
these  two  forces  alone  prevailed,  the  ball  d 
would  proceed  in  the  dotted  line  to  B. 
But  as  the  resistance  of  the  air  operates  in 
direct  opposition  to  the  force  of  projection, 
instead  of  reaching  the  ground  at  B,  the  ^ 
ball  will  fall  somewhere  about  E.^ 

64.  When  a  body  is  thrown  in  a  horizontal  direc- 
tion, or  upwards  or  downwards  obliquely,  its  course  will 

*  It  is  calculated  that  the  resistance  of  the  air  to  a  cannon-ball  of  two 
pounds  weight,  with  the  velocity  of  two  thousand  feet  in  a  second,  is  more 
than  equivalent  to  sixty  times  the  weight  of  the  ball. 


63.  What  is  a  projectile?  What  lines  do  projectiles  describe?  From 
what  cause?  Give  the  illustration.  How  great  is  the  resistance  of  the  air 
calculated  to  be  to  a  cannon-ball  of  two  pounds  weight,  with  the  velocity 
of  2000  feet  in  a  second  ? 


MECHANICS. 


45 


be  in  the  direction  of  a  curve-line,  called 
Si  parabola  f  {see  Fig.  15,)  but  when  it  is 
thvo\vnpe7ye7idicularlyupwa.Ydsordown' 
wards,  it  will  move  perpendicularly,  be- 
cause the  force  of  projection  and  that  of 
gravity  are  in  the  same  line  of  direction.  ^ 

*  The  science  of  gunnery  is  founded  upon  the  laws  relating  to  projec- 
tiles. The  force  of  gunpowder  is  accurately  ascertained,  and  calculations 
are  predicated  upon  these  principles,  which  enable  the  engineer  to  direct  his 
guns  in  such  a  manner  as  to  cause  the  fall  of  the  shot  or  shells  in  the  very 
spot  where  he  intends.  The  knowledge  of  this  science  saves  an  immense 
expenditure  of  ammunition,  which  would  otherwise  be  idly  wasted  without 
producing  any  effect.  In  attacks  upon  towns  and  fortifications,  the  skilful 
engineer  knows  the  means  he  has  in  his  power,  and  can  calculate,  with 
great  precision,  their  effects.  It  is  in  this  way  that  the  art  of  war  has 
been  elevated  into  a  science,  and  much  is  made  to  depend  upon  skill, 
which,  previous  to  the  knowledge  of  these  principles,  depended  entirely 
upon  physical  power. 

The  force  with  which  balls  are  thrown  by  gunpowder  is  measured  by  an 
mstrument  called  the  Ballistic  pendulum.  It  consists  of  a  large  block  of 
wood  suspended  by  a  rod  in  the  manner  of  a  pendulum.  Into  this  block 
the  balls  are  fired,  and  to  it  they  commuiiicate  their  own  motion.  Now 
the  weight  of  the  block  and  that  of  the  ball  being  known,  and  the  motion 
or  velocity  of  the  block  being  determined  by  machinery,  or  by  observation, 
the  elements  are  obtained  by  which  the  velocity  of  the  ball  may  be  found  ; 
for,  the  weight  of  the  ball  is  to  the  weight  of  the  block  as  the  velocity  of 
the  block  is  to  the  velocity  of  the  ball.  By  this  simple  apparatus,  many 
facts  relative  to  the  art  of  gunnery  may  be  ascertained.  If  the  ball  be 
fired  from  the  same  gun,  at  different  distances,  it  will  be  seen  how  much 
resistance  the  atmosphere  opposes  to  its  force  at  such  distances.  Rifles 
and  guns  of  smooth  bores  may  be  tested,  as  well  as  the  various  charges  of 
powder  best  adapted  to  different  distances  and  different  guns.  These,  and 
a  great  variety  of  other  experiments,  useful  to  the  practical  gunner  or 
sportsman,  may  be  made  by  this  simple  means. 

The  velocity  of  balls  impelled  by  gunpowder  from  a  musket  with  a 
common  charge,  has  been  estimated  at  about  1650  feet  in  a  second  of  time, 
when  first  discharged.  The  utmost  velocity  that  can  be  given  to  a  can- 
non-ball, is  2000  feet  per  second  ;  and  this  only  at  the  moment  of  its  leav- 
ing the  gun. 

In  order  to  increase  the  velocity  from  1650  to  2000  feet,  one  half  more 

64.  When  a  body  is  thrown  horizontally,  or  upwards  or  downwards 
obliquely,  in  what  curve  will  it  move  ?  In  what  hne  will  it  move  when 
thrown  upwards  or  downwards  obliquely  ? 


46  NATURAL  PHILOSOPHY. 

65.  The  random  of  a  projectile  is  the  horizontal  dis- 
tance from  the  place  whence  it  is  thrown,  to  the  place 
where  it  strikes.  The  greatest  random  takes  place  at  an 
angle  of  45  degrees— that  is,  w^hen  a  gun  is  pointed  at 
this  angle  with  the  horizon,  the  ball  is 

thrown  to  the  greatest  distance. 

Let  Fig.  16  represent  a  gun  or  a  carron- 
ade,  fron?  which  a  ball  is  thrown  at  an  an- 
gle of  45  degrees  with  the  horizon.  If  the 
ball  be  thrown  at  any  angle  above  45  de- 
grees, the  random  will  be  the  same  as  it 
would  be  at  the  same  number  of  degrees 
below  45  degrees.* 

66.  When  the  centre  of  gravity  of  a  body  is  support- 
ed, the  body  itself  will  be  supported ;  but^  when  the 
centre  of  gravity  is  unsupported,  the  body  wiU  falLf 

powder  is  required  ;  and  even  then,  at  a  long  shot,  no  advantage  is  gained  ; 
since,  at  the  distance  of  500  yards,  the  greatest  velocity  that  can  be  ob- 
tained is  only  1200  or  1300  feet  per  second.  Great  charges  of  powder  are 
therefore  not  only  useless,  but  dangerous;  for,  though  they  give  little  ad- 
ditional force  to  the  ball,  they  hazard  the  lives  of  many  by  their  liability 
to  burst. 

Experiment  has  also  shown,  that,  although  long  guns  give  a  greater  ve- 
locity to  the  shot  than  short  ones,  still,  that  on  the  whole,  short  ones  are 
preferable  ;  and,  accordingly,  armed  ships  are  now  almost  invariably  fur- 
nished with  short  guns,  called  carronades. 

The  length  of  sporting  guns  has  also  been  gi-eatly  reduced,  of  late  years. 
Formerly,  the  barrels  were  from  four  to  six  feet  in  length  ;  but  the  best 
fowUng-'pieces  of  the  present  day  have  barrels  of  two  feet,  or  two  and  a 
half,  only,  in  length.  Guns  of  about  this  length  are  now  universally  em- 
ployed for  such  game  as  woodcocks,  partridges,  grouse,  and  such  birds  as 
are  taken  on  the  wing,  with  the  exceptions  of  ducks  and  wild  geese,  which 
require  longer  and  heavier  guns. 

*  A  knowledge  of  this  fact,  and  calculations  predicated  on  it,  enables 
the  engineer  so  to  direct  his  guns,  as  to  reach  the  object  of  attack  when 
within  the  range  of  shot. 

t  The  Boston  School  Apparatus  contains  a  set  of  eight  illustrations  for 

65.  What  is  the  random  of  a  projectile  ?  At  what  angle  does  the  great- 
est random  take  place? 

66.  When  the  centre  of  gravity  of  a  body  is  supported,  will  the  body 
stand  or  fall  ?    What  if  the  centre  be  unsupported  ? 


MECHANICS. 


47 


A  line  drawn  from  the  centre  of  gravity,  perpendicu- 
lar to  the  horizon,  is  called  the  line  of  direction.'* 

67.  When  the  line  of  direction  falls  within  the  basef 
of  any  body,  the  body  will  stand  ;  but  when  that  line 
falls  outside  of  the  base,  the  body  will  fall  or  be  overset. 

1.  Fig.  18  represents  a  loaded  wagon  on  the  dedivity  of  a 
hill.    The  hne  C  F  represents  a  horizontal  line, 

D  E  the  base  of  the  wagon.  If  the  w^agon  be  ^^s-  is. 
loaded  in  such  a  manner  that  tlie  centre  of 
gravity  be  at  B,  the  perpendicular  B  D  will  fall 
within  the  base,  and  the  wagon  will  stand. 
But  if  the  load  be  altered  so  that  the  centre  of 
gravity  be  raised  to  A,  the  perpendicular  A  C 
wall  fall  outside  of  the  base,  and  the  wagon 
will  be  overset.  From  this  it  follow^s  that  a  wagon,  or  any 
carriage,  will  be  most  firmly  supported  when  the  line  of  direction 
of  the  centre  of  gravity  falls  exactly  between  the  w^heeis  ;  and 
that  is  the  case  on  a  level  road.  The  centre  of  gravity  in  the 
human  body,  is  between  the  hips,  and  the  base  is  the  feet. 

So  long  as  we  stand  uprightly,  the  line  of  direction  falls 
w^ithin  this  base.  When  we  lean  on  one  side,  the  centre  of 
gravity  not  being  supported,  we  no  longer  stand  firmly. 

2.  A  rope-dancer  performs  all  his  feats  of  agility,  by  dex- 
terously supporting  the  centre  of  gravity.  For  this  purpose 
he  carries  a  heavy  pole  in  his  hands,  which  he  shifts  from  side 
to  side  as  he  alters  his  position,  in  order  to  throw  the  weight 
to  the  side  which  is  deficient;  and  thus,  by  changing  the 

the  purpose  of  giving  a  clear  idea  of  the  centre  of  gravity,  and  showing 
the  difference  between  the  centre  of  gravity  and  the  centre  of  magnitude. 

*  The  line  of  direction  is  the  line  which  the  centre  of  gravity  would 
describe  if  the  body  were  permitted  to  fall. 

t  The  base  of  a  body  is  its  lowest  side.   The  base  Fig.  17. 

of  a  body  standing  on  wheels  or  legs,  is  represent- 
ed  by  lines  drawn  from  the  lowest  part  of  one 
wheel  or  leg,  to  the  lowest  part  of  the  other  wheel 
or  leg. 

Thus,  m  Figs.  17  and  18,  D  E  represents  the  base 
of  the  wagon  and  of  the  table. 

What  is  the  line  of  direction  ? 

67.  If  the  line  of  direction  falls  within  the  base,  will  the  body  stand  or 
fall?    Give  an  illustration. 


48 


NATURAL  PHILOSOPHY. 


situation  of  the  centre  of  grai-ity,  lie  keeps  the  hne  of  dn-ec- 
tion  ^-iihin  the  base,  and  he  will  not  fah.^ 

A  spherical  body  will  roll  down  a  slope,  because  the  centre 
of  gravity  is  not  supported. f 

68.  When  a  bodv  is  of  uniform  density,  the  centre 
of  gravity  is  in  the  same  point  ^vith  the  centre  of  mag- 
nitude. .  1    r  1_  • 

When  one  part  of  the  body  is  composed  ot  heavier 
materials  than  another  part,  the  centre  of  gravity  (being 
the  centre  of  the  weight  of  the  body)  no  longer  corre- 
sponds with  the  centre  of  magnitude.  Thus,  the  centre 
of  gravity  of  a  cylinder  plugged  with  lead,  is  not  in  the 
same  point  as  the  centre  of  magnitude. 

Bodies,  therefore,  consisting  of  but  one  kind  of  substance,  as 
wood,  stone,  or  lead,  and  whose  densities  are  consequently 
uniform,  will  stand  more  tirmly  than  bodies  composed  of  a 
varietv  of  substances,  of  different  densities. 

*  The  shepherds  in  the  south  of  France  alFord  an  interesting  instance 
of  the  appUcation  of  the  art  of  balancing  to  the  common  business  of  hfe. 
These  men  walk  on  stilts  from  three  to  four  feet  high,  and  their  children, 
when  quite  voung,  are  taught  to  practise  the  same  art.  By  means  of 
these  odd  additioi  to  the  length  of  the  leg,  their  feet  are  kept  out  of  the 
water,  or  the  heated  sand,  and  they  are  also  enabled  to  see  their  sheep  at 
a  greater  distance.  They  use  these  stilts  with  great  skill  and  care,  and 
run,  jump,  and  even  dance  on  them  with  great  ease. 

t  A  cyhnder  can  be  made  to  roll  up  a  slope,  by  plugging  one  side  of  it 
with  lead  :  the  body  being  no  longer  of  a  uniform  density,  the  centre  of 
gravity  is  removed  from  the  middle  of  the  body  to  some  point  in  the  lead, 
as  that  substance  is  much  heavier  than  wood.  Xow,  in  order  that  the 
cyhnder  mav  roll  down  the  plane,  as  it  is  here  situated,  the  centre  of  grav- 
ity must  rise,  which  is  impossible  ;  the  centre  of  gravity  must  always  de- 
scend in  moving,  and  ^vill  descend  by  the  nearest  and  readiest  means, 
which  wiU  be  by  forcing  the  cylinder  up  the  slope,  until  the  centre  of 
OTavitv  is  supported,  and  then  it  stops. 

4  body  also  in  the  shape  of  two  cones  united  at  their  bases,  can  be 
made  to  roll  up  an  incUned  plane  formed  by  two  bars  with  their  lower  ends 
inclined  towards  each  other.  This  is  iUustrated  by  a  simple  contrivance  m 
the  "  Boston  School  Set,"  and  the  fact  illustrated  is  called  "  the  mechani- 
cal paradox."    

68  If  a  body  is  of  uniform  density,  what  centres  will  coincide?  Give 
an  example  in  which  the  centres  of  magnitude  and  gravity  will  not  coin- 
cide 


MECHANICS. 


49 


69.  Bodies  that  have  a  narrow  base  are  easily  over- 
set ;  for  if  they  are  in  the  least  degree  incHned,  the  hne 
of  direction  will  fall  outside  of  the  base,  and  their  cen- 
tre of  gravity  will  not  be  supported.* 

The  broader  the  base,  and  the  nearer  the  centre  of 
gravity  to  the  ground,  the  stronger  will  be  the  edifice. 

For  this  reason  a  pyramid,f  having  a  broad  base  and  but 
little  elevation,  is  the  firmest  of  all  structures. 

70.  When  two  bodies  are  fastened  together,  they  are 
to  be  considered  as  forniing  but  one  body,  and  have  but 
one  centre  of  gravity.  If  the  two  bodies  be  of  equal 
weight,  the  centre  of  gravity  will  be  in  the  middle  of 
the  line  which  unites  them.  But  if  one  be  heavier  than 
the  other,  the  centre  of  gravity  will  be  as  much  nearer 
to  the  heavier  one  as  the  heavier  exceeds  the  light  one 
in  weight. 

1.  Fig.  19  represents  a  bar  with  an 

equal  weight  fastened  at  each  end:  the  ^ig-  ^9. 

centre  of  gravity  is  at  A,  the  middle  of  g   ^  ^ 

the  bar,  and  whatever  supports  this 
centre  will  support  both  the  bodies  and 
the  pole. 

2.  Fig.  20  represents  a  bar  with  an 
unequal  weight  at  each  end.  The  cen- 
tre of  gravity  is  at  C  nearer  to  the  larger 
body. 

*  A  person  can  carry  two  pails  of  water  more  easily  than  one,  because 
the  pails  balance  each  other,  and  the  centre  of  gravity  remains  supported 
by  the  feet.  But  a  s'm<y\e  pail  throws  the  centre  of  gravity  on  one  side, 
and  renders  it  more  difficult  to  support  the  body. 

t  A  cone  has  also  the  same  degree  of  stability  ;  but,  strictly  speaking, 
a  cone  is  a  pyramid  with  an  infinite  number  of  sides. 

69.  What  shaped  bodies  are  easily  overturned  ?  What  bodies  must  stand 
more  firmly  than  others?  Why?  Why  do  bodies  which  have  a  narrow 
base  overturn  more  easily  than  those  which  have  broad  bases?  Why  can 
a  person  carry  two  pails  of  water  more  easily  than  one?  Why  is  a  pyra- 
mid the  firmest  of  all  structures  ? 

70.  If  two  bodies  of  equal  weight  are  fastened  together,  where  is  tho 
centre  of  gravity  ?  If  one  be  heavier  than  the  other  ?  What  does  Fig. 
19  represent?    Fig.  20 ?    Fig.  21  ? 

3 


Fig.  20. 


50  NATURAL  PHILOSOPHY. 

3.  Fio".  21  represents  a  bar  with  un- 
equal weights  at  each  end,  but  the  larger 
weight  exceeds  the  less  in  such  a  degree 
thaf  the  centre  of  gravity  is  within  the 
larger  body  at  C. 


RESULTANT  MOTION. 


Fig.  21. 


71.  Resultant  motion  is  the  efFect  or  result  of  two 
motions  compounded  into  one. 

If  two  men  be  sailing  in  separate  boats,  in  the  same  direc- 
tion, and  at  the  same  rate,  and  one  toss  an  apple  to  the  other, 
the  apple  would  appear  to  pass  directly  acioss  from  one  to 
the  other,  m  a  line  of  direction  perpendicular  to  the  side  of 
each  boat.  But  its  real  course  is  through  the  air  in  the 
diagonal  of  a  parallelogram,  fonned  by  the  hues  representing 
the°  course  of  each  boat,  and  perpendiculars  drawn  to  those 
lines  from  the  spot  where  each  man  stands  as  the  one  tosses 
and  the  other  catches  the  apple.  In  Fig.  22  ^. 
the  lines  A  B  and  C  D  represent  the  course  'J'  ^  ' 

of  each  boat;  E  the  spot  where  the  man     ^   —  g 

stands  who  tosses  the  apple ;  while  the  apple     ^     [  /  |  ^ 
is  in  its  passage,  the  boats  have  passed  from     ^     e  h 
E  and  G,  to  H  and  F  respectively.    But  the 
apple  ha\'ing  a  motion,  with  the  man,  that  would  carry  it  from 
E  to  H  and  hkewise  a  projectile  force  which  would  carry  it 
from  E  to  G,  cannot  obey  them  both,  but  will  pass  through 
the  dotted  Ime  E  F,  which  is  the  diagonal  of  the  parallelogram 
E  G  F  H.^ 

*  On  the  principle  of  resultant  motion,  if  two  ships  in  an  engagement 
be  sailino-  before  the  wind,  at  equal  rates,  the  aim  of  the  gunners  will  be 
exactly  as  though  they  both  stood  stiU.  But  if  the  gunner  fire  from  a  ship 
standing  still,  at  another  under  sail,  or  a  sportsman  fire  at  a  bird  on  the 
wing,  each  should  take  his  aim  a  little  forward  of  the  mark,  because  the 
ship  and  the  bird  will  pass  a  little  forward  while  the  shot  is  passing  to 
them. 


71.  What  is  resultant  motion?  Give  the  examples  of  this  kind  of  mo- 
tion. 


MECHANICS. 


51 


THE  PENDULUM. 

72.  The  Pendulum*  consists  of  a  weight  or  ball  sus- 
pended by  a  rod,  and  made  to  swing  backwards  and 
forwards. 

73.  The  motions  of  a  pendulum  are  called  its  vibra- 
tions, and  they  are  caused  by  gravity.f  The  part  of  a 
circle  through  which  it  moves,  is  called  its  arc. 

74.  The  vibrations  of  pendulums  of  equal  length,  are 

*  The  pendulum  was  invented  by  Galileo,  a  great  astronomer  of  Flor- 
ence, in  the  beginning  of  the  seventeenth  century.  Perceiving  that  the 
chandeliers  suspended  from  the  ceiling  of  a  lofty  church  vibrated  long  and 
with  great  uniformity,  as  they  were  moved  by  the  wind  or  by  any  acci- 
dental disturbance,  he  was  led  to  inquire  into  the  cause  of  their  motion, 
and  this  inquiry  led  to  the  invention  of  the  pendulum.  From  a  like  appa- 
rently insignificant  circumstance  arose  the  great  discovery  of  the  principle 
of  gravitation.  During  the  prevalence  of  the  plague,  in  the  year  1665, 
Sir  Isaac  Newton  retired  into  the  country  to  avoid  the  contagion.  Sitting 
in  his  orchard,  one  day,  he  observed  an  apple  fall  from  a  tree.  His  in- 
quisitive mind  was  immediately  led  to  consider  the  cause  which  brought 
the  apple  to  the  ground,  and  the  result  of  his  inquiry  was  the  discovery 
of  that  grand  principle  of  gravitation,  which  may  be  considered  as  the  first 
and  most  important  law  of  material  nature.  Thus,  out  of  what  had  been 
before  the  eyes  of  men,  in  one  shape  or  another,  from  the  creation  of  the 
world,  did  these  philosophers  bring  the  most  important  results. 

t  When  a  pendulum  is  raised  from  its  perpendicular  position,  its  weight 
will  cause  it  to  fall,  and,  in  the  act  of  falling,  it  acquires  a  degree  of  mo- 
tion which  impels  it  to  a  height  beyond  the  perpendicular  almost  as  great 
as  that  to  which  it  was  raised.  Its  motion  being  thus  spent,  gravity  again 
acts  upon  it  to  bring  it  to  its  original  perpendicular  position,  and  it  again 
acquires  an  impetus  in  falling  which  carries  it  nearly  as  high  on  the  oppo- 
site side.  It  thus  continues  to  swing  backwards  and  forwards,  until  the 
resistance  of  the  air  wholly  arrests  its  motion.  In  the  construction  of 
clocks,  an  apparatus  connected  with  the  weight  or  the  spring  is  made  to 
act  on  the  pendulum  with  such  a  force,  as  to  enable  it  to  overcome  the 
resistance  of  the  air,  and  keep  up  a  continued  motion. 


72.  Of  what  does  a  pendulum  consist  ?  By  whom  was  the  pendulum 
invented ?  What  led  him  to  the  discovery?  By  whom  was  the  principle 
of  gravitation  discovered  ?    What  led  him  to  the  discovery  ? 

73.  What  are  the  movements  of  the  pendulum  called  ?  What  is  meant 
by  its  arc?    What  causes  its  vibrations? 


52  NATURAL  PHILOSOPHY. 

very  nearly  equal,  whether  they  move  through  a  greater 
or  less  part  of  their  arcs. 

In  Fig.  23,  A  B  represents  a  pendulum. 
D  F  E  C  the  arc  in  Avhich  it  vibrates.  If  the 
pendulum  be  raised  to  E  it  will  return  to  F, 
if  it  be  raised  to  C  it  will  return  to  D  m 
nearly  the  same  length  of  time,  because  that 
in  proportion  as  the  arc  is  more  extended,  the 
steeper  will  be  its  beginnings  and  endings, 
and,  therefore,  the  more  rapidly  will  it  taU. 

75  The  time  occupied  in  the  vibration  of  a  pendu- 
lum, depends  upon  its  length.  The  longer  the  pendu- 
lum,  the  slower  are  its  vibrations. 

76  The  leno-th  of  a  pendulum  which  vibrates  sixty 
times  in  a  minute  (or,  in  other  words  which  vibmtes 
seconds)  is  about  89  inches.  But  m  different  parts  ot 
the  earth  this  length  must  be  varied.  A  pendulum  to 
vibrate  seconds  at  the  equator  must  be  shorter  than  one 
which  vibrates  seconds  at  the  poles."" 

77  A  clock  is  regulated  by  lengthening  or  shorten- 
W  the  pendulum.  By  lengthening  the  pendulum  the 
clock  is  made  to  go  slower ;  by  shortening  it,  it  will  go 
faster.f 

^  The  equatorial  diameter  of  the  earth  exceeds  the  polar  diameter  by 
about  34  miles;  consequently,  the  poles  must  be  nearer  to  the  centre  of 
the  earth's  attraction  than  the  equator,  and  gravity  must  also  operate  with 
greater  force  at  the  poles  than  at  the  equator.  Hence,  also,  the  length  of 
a  pendulum,  to  vibrate  in  any  given  time,  must  vary  with  the  latitude  of 

the  place.  ,  ,  „ 

t  The  pendulum  of  a  clock  is  made  longer  or  shorter,  by  means  of  a 

74.  HoWo  the  vibrations  of  pendulums  of  equal  length  compare? 

Illustrate  by  Fig.  23.  j  i      j       j  7 

75  Upon  what  does  the  time  of  the  vibrations  of  a  pendulum  depend? 

76  What  is  the  length  of  a  pendalum  which  vibrates  sixty  times  in  a 
minute^  Do  different  situations  affect  the  vibrations?  How  can  a  pen- 
dulum which  vibrates  seconds  at  the  equator  be  made  to  vibrate  seconds 

7?  How  is  a  clock  regulated  ?  What  effect  has  the  lengthening  of  the 
pendulum?  The  shortening?  What  is  a  clock?  Of  what  use  is  the 
weight  ?  What  do  the  wheels  show  ?  Why  do  clocks  go  siower  in  sum- 
mer than  in  winter  ?    How  does  a  watch  differ  from  a  clock  ? 


MECHANICS. 


53 


THE  MECHANICAL  POWERS. 

78.  The  Mechanical  Powers  are  certain  contrivances 
designed  to  increase  or  to  diminish  force,  or  to  alter  its 
direction. 

There  are  five  things  in  mechanics  w^hich  require  a 
distinct  consideration,  namely: 
First,  the  power  that  acts. 

Secondly,  the  resistance  which  is  to  be  overcome  by 
the  power. 

Thirdly,  the  centre  of  motion,  or,  as  it  is  sometimes 
called,  the  fulcrum.* 

Fourthly,  the  respective  velocities  of  the  power  and 
the  resistance  ;  and. 

Fifthly,  the  instruments  employed  in  the  construction 
of  the  machine. 

1.  The  power  that  acts  is  the  muscular  strength  of  men,  or 

screw  beneath  the  weight  or  ball  of  the  pendulum.  The  clock  itself  is 
nothing  more  than  a  pendulum  connected  with  wheel-work,  so  as  to  re- 
cord the  number  of  vibrations.  A  weight  is  attached,  in  order  to  counter- 
act the  retarding  effect  of  friction,  and  the  resistance  of  the  air.  The 
wheels  show  how  many  swings  or  beats  of  the  pendulum  have  taken  place 
in  a  given  time,  because,  at  every  beat,  the  tooth  of  a  wheel  is  allowed  to 
pass.  Now  if  this  wheel  have  sixty  teeth,  it  will  turn  round  once  in  sixty 
vibrations  of  the  pendulum,  or  in  sixty  seconds  :  and  a  hand,  fixed  on  the 
axis  of  the  wheel  projecting  through  the  dial-plate,  will  be  the  second-hand 
of  the  clock.  Other  wheels. are  so  connected  with  the  first,  and  the  num- 
ber of  teeth  in  theni  is  so  proportioned,  that  the  second-wheel  turns  sixty 
times  slower  than  the  first,  and  to  this  is  attached  the  minute-hand  ;  and 
the  third  wheel,  moving  twelve  times  slower  than  the  second,  carries  the 
hour-hand.  On  account  of  the  expansion  of  the  pendulum  by  heat,  and 
its  contraction  by  cold,  clocks  will  go  slower  in  summer  than  in  winter, 
because  the  pendulum  is  thereby  lejigthened  at  that  season. 

A  watch  differs  from  a  clock,  in  having  a  vibrating  wheel  instead  of  a 
pendulum.  This  wheel  is  moved  by  a  spring,  called  the  hair-spring. 
The  place  of  the  weight  is  supplied  by  another  larger  spring,  called  the 
main-'-jjring. 

*  The  word  fulcrum  means  a  prop,  or  support. 

78.  What  are  the  mechanical  powers  ?  How  many  things  are  to  be 
considered  in  order  to  understand  the  power  of  a  machine  ?  What  is  the 
first?    Second?    Third?    Fourth?  Fifth? 


54 


NATURAL  PHILOSOPHY. 


animals,  the  weight  and  momentum  of  solid  bodies,  the  elastic 
force  of  steam,  springs,  the  pressure  of  the  air,  &c. 

2.  The  resistance  to  be  overcome  is  the  attraction  of  gravity, 
or  of  cohesion,  the  inertness  of  matter,  &c. 

3.  The  centre  of  motion,  or  the  fulcrum,  is  the  point  about 
which  all  the  parts  of  the  body  move. 

4.  The  velocity  is  the  rapidity  with  which  an  effect  is  pro- 
duced. 

5.  The  instruments  are  the  mechanical  powers  which  enter 
into  the  construction  of  the  machine.* 

79.  There  are  six  mechanical  powers,  namely,  the 
Lever,  the  Pulley,  the  Wheel  and  Axle,  the  Inclined 
Plane,  the  Wedge,  and  the  Screw. 

80.  The  Leverf  is  an  inflexible  bar,  moveable  on  a 
fulcrum,  or  prop. 

There  are  three  kinds  of  levers,  called  the  first,  sec- 
ond, and  third  kinds,  according  to  the  respective  position 
of  the  fulcrum,  the  power,  and  the  weight. 

81.  In  a  lever  of  the  first  kind,  the  weight  is  at  one  end, 
the  power  at  the  other,  and  the  fulcrum  between  them. 

Fig.  24  represents  a  lever  of  the  first  kind,  resting  on  the 
fulcrum  F,  and  moveable  upon  it.  W  is  the  weight  to  be 
moved,  and  P  is  the  power  which 
moves  it.  The  advantage  gained  in 
raising  a  iveight  hy  the  use  of  this  kind  b 
of  lever  y  is  in  proportion  as  the  distance 
of  the  power  from  the  fulcrum  exceeds 
that  of  the  weight  from  the  fulcrum. 
Thus,  in  this  figure,  if  the  distance 
between  P  and  F  be  double  that  be- 

*  All  machines  and  instruments  are  constructed  on  the  principle  of 
some  one  or  more  of  the  mechanical  powers. 

t  The  lever  is  made  in  a  great  variety  of  forms,  and  of  many  different 

What  is  the  power  that  acts  ?  What  is  the  resistance  to  be  overcome  ? 
What  is  the  fulcrum?    What  is  the  velocity  ? 

79.  How  many  mechanical  powers  are  there  ?    What  are  they? 

80.  What  is  a  lever?  How  many  kinds  of  levers  are  there  ?  How  do 
they  differ? 

81.  What  is  a  lever  of  the  first  kind?  What  figure  illustrates  this? 
Explain  it  by  the  figure.  To  what  is  the  advantage  gained  by  this  lever 
proportional  ? 


MECHANICS. 


55 


tween  W  and  F,  then  a  man,  by  the  exertion  of  a  foi  ce  of  100 
pounds  with  the  lever,  can  move  a  weight  of  200  pounds.  From 
this  it  follows  that  the  nearer  the poiuer  is  applied  to  the  end  of  U>c 
h'rer,  the  greater  is  the  advantage  gained.  Thus,  a  greater 
w<^ight  can  be  moved  by  the  same  power,  when  applied  at  IJ, 
ii\:in  when  it  is  exerted  at  P.'^ 

2.  Tiie  common  steelyard,  an  instrument  for  weighing  av ti- 
des, is  constructed  on  the  principle  of  the  lever  of  the  ii.bt 
kind.  It  consists  of  a  rod  or  bar,  marked  with  notches  to 
designate  the  pounds  and  ounces,  and  a  weight  which  is  move- 
able\long  the  notches.   The  bar  is  furnished  with  three  hook  s 

materials  ;  and  is  much  used  in  almost  every  kind  of  mechanical  operation. 
Sometimes  it  is  detached  from  the  fulcrum,  but  most  generally  the  fulcrum 
is  a  pin  or  rivet  by  which  the  lever  is  permanently  connected  with  the 
iramework  of  other  parts  of  the  machinery. 

*  It  is  a  fundamental  principle  in  mechanics,  that  what  is  gained  in 
power  is  lost  in  time.    To  illustrate  this  principle,  (Fig.  25,)  W  represents 
the  weight,  F  the  fulcrum,  P  the  power,  and  the 
bar  W  F  P  the  lever.    To  raise  the  weight  W  to  Fig.  25. 

ID,  the  power  P  must  descend  to  p.  But  as  the 
radius  of  the  circle  in  which  the  power  P  moves 
is  double  that  of  the  radius  of  the  circle  in  which 
the  weight  W  moves,  the  arc  P  p  is  double  the 
arc  w  ;  or,  in  other  words,  the  distance  P  ^  is 
double  the  distance  of  W  to.  Now,  as  these  dis- 
tances are  traversed  in  the  same  time  by  the 
power  and  the  weight  respectively,  it  follows, 

that  the  velocity  of  the  power  must  be  double  the  velocity  of  the  weight ; 
that  is,  the  power  must  move  at  the  rate  of  two  feet  in  a  second,  in  order 
to  move  the  weight  one  foot  in  the  same  time. 

This  principle  applies  not  only  to  the  lever,  but  to  all  the  mechanical 
powers,  and  to  all  machines  constructed  on  mechanical  principles. 

When  two  weights  are  equal,  and  the  fulcrum  is  placed  exactly  in  the 
centre  of  the  lever  between  them,  they  will  mutually  balance  each  other; 
or,  in  other  words,  the  centre  of  gravity  being  supported,  neither  of  the 
weights  will  sink.   This  is  the  principle  of  the  common  scale  for  weighing. 

To  gain  power  by  the  use  of  the  lever,  the  fulcrum  must  be  placed  near 
the  weight  to  be  moved,  and  the  power  at  the  greater  distance  from  it. 
The  force  of  the  lever,  therefore,  depends  on  its  length,  together  with 
the  power  applied,  and  the  distance  of  the  weight  from  the  f  ulcrum. 

What  follows  from  this?  What  is  meant  by  an  indexible  bar?  Note 
What  is  a  fundamental  principle  in  Mechanics?  Illustrate  this  by  the 
figure.    Does  this  principle  ^ppsy  to  all  the  mechanical  powers  ? 


NATURAL  PHILOSOPHY. 


on  the  longest  of  which,  the  article  to  he  v:eighed  is  always  to  he 
hung.    The  other  two  hooks  serve  for  the  handle  of  the  in- 


stiTiment  when  in  use.  The  pivot  of  each  of  these  two  hooks 
serves  for  the  ftilcrtini.  When  suspended  by  the  hook  C,  as  in 
Fi^.  26,  it  is  manifest  that  a  pound  weight  at  E  will  balance  as 
manv  pounds  at  W,  as  the  distance  between  the  pivot  of  D,  and 
the  pivot  of  C,  is  contained  in  the  space  between  the  pivot  of 
C  and  the  ring  from  which  E  is  suspended. 


MECHANICS. 


57 


3.  The  same  instrument  may  be  used  to  weigh  heavy  arti- 
cles, by  using  the  middle  hook  for  a  luindle,  where,  as  will  be 
seen  in  the  figure,  the  space  between  the  pivot  of  F  (which  in 
this  case  is  the  fulcrum)  and  tlie  pivot  of  D  (from  which  the 
w light  is  suspended)  being  lessened,  is  contained  a  greater 
lumber  of  times  in  the  distance  between  the  fulcrum  and  the 
notches  on  the  bar.  The  steelyard  is  furnished  with  two  sets 
of  notches  on  opposite  sides  of  the  bar.  An  equilibrium  will 
always  be  produced  when  the  product  of  the  weights  on  the 
opposite  sides  of  the  fulcrum  into  their  respective  distances 
from  it,  are  equal  to  one  another. 

4.  A  balance,  or  pair  of  scales,  is  a  lever  of  the  first  kind, 
vi  h  equal  arms.  Steelyards,  scissors,  pincers,  snuffers,  arid  a 
poker  used  for  stirring  the 

r  11    1  ^  4-U  Fig.  28. 

fire,  are  all  levers  oi  the     ^  ^   c 

first  kind.  The  longer  the 
handles  of  scissors,  pincers, 
&c.,  and  the  shorter  the 
points,  the  more  easily  are 

they  used.  A  compound  lever,  represented  in  Fig.  28,  consists 
of  several  levers,  so  arranged  that  the  shorter  arm  of  one  may 
act  on  the  longer  arm  of  the  other.  Great  power  is  obtained 
in  this  way ;  but  its  exercise  is  limited  to  a  very  small  space. 

82.  In  a  lever  of  the  second  kind,  the  fulcrum  is  at 
one  end,  the  power  at  the  other,  and  the  weight  between 
them. 

1.  Let  Fig.  29  represent  a  lever  of  the  second  kind.   F  is  the 
fulcrum,  P  the  power,  and  W  the  weight.  j,.^  29. 
The  advantage  gained  by  a  lever  of  this  , 

kind  is  in  proportion  as  the  distance  of  the    P^/  p 
power  from  the  fulcrum  exceeds  that  of  the  .  ,  , 

weight  from  the  fulcrum.    Thus  in  this  fig-  I 
ure,  if  the  distance  from  P  to  F,  is  four  times  ^ 
the  distance  from  W  to  F,  then  a  power  of 
one  pound  at  P  will  balance  a  weight  of  four  pounds  at  W. 

2.  On  the  principle  of  this  kind  of  lever,  two  persons  carry- 


Explain  the  common  steelyard  in  Fig.  26.  Also  in  Fig.  27.  What  is  a 
balance,  or  pair  of  scales?    Give  some  examples  of  levers  of  the  first  kind. 

82.  What  is  a  lever  of  the  second  kind?  What  figure  illustrates  this? 
To  what  is  the  advantage  gained  by  this  lever  proportional  ?  Give  some 
examples  of  levers  of  the  second  kind. 


59  NATURAL  PHILOSOPHY. 

iog  a  beaTV  bnrden,  suspended  on  a  bar,  may  be  made  to  bear 
unequal  po'rtions  of  it,  by  placing  it  nearer  to  the  one  than  the 
other. 

3.  Two  horses,  also,  mar  be  made  to  draw  unequal  portions 
<rf  a"  load,  bv  dividing  the  bar  anached  to  the  carriage  in  such 
a  manner  that  the  weaker  horse  may  draw  upon  the  longt^r 

end  of  it.  i  -  j 

4.  Oars,  rudders  of  ships.  d.>Drs  turning  on  hmges,  and 
cuttincr-kniTes,  which  are  hxed  at  one  end,  are  constructed  up- 
on the"  principle  of  levers  of  the  second  kind.* 

83.  In  a  lever  of  the  third  kind,  the  fulcrum  is  at  one 
end,  the  weight  at  the  other,  and  the  power  is  applied 
between  them. 

1.  In  levers  of  this  kind  the  powar  must  always  axeed  the 
weiqfit,  in  the  .^ime  proportion  as  the  distance  of  the  weight  from 
the  fulcrum  exceeds  that  of  the  power  from  the  fulcrum. 

2.  In  Fig.  30,  F  is  the  fulcrum,  W  the  weight,  and  P  the 
power  between  the  fulcrum  and  the  weight ;  ^ 
and  the  piwer  must  exceed  the  weight  in  the  " 
same  proportion  that  the  distance  between  W    p  ^ p 

and  F  exceeds  the  distance  between  P  and  F.      JL  . 

3.  A  laddt-r  which  is  to  be  raised  by  the 

stren^^th  of  a  man's  arms,  represents  a  lever  ^ 
of  this  kind,  where  the  fulcrum  is  that  end  w 
which  is  fixed  against  the  wall :  the  weight 
may  be  considered  as  at  the  top  part  of  the  ladder,  and  tb- 
power  is  the  strength  apphed  in  raismg  it. 

4.  The  bDnes  of  a  man's  arm,  and  most  of  the  moveable 
bones  of  animals,  are  levers  of  the  tlurd  kind.  But  the  loss  of 
power  m  hmbs  of  anim-Js  is  compensated  by  the  beautv  and 
compactness  ::  : V  e  liii:  ?.  as  weU  as  the  increased  velocity  of 
their  mo:!:::.  Tz-  -z-^-s  m  clock  and  watch  work,  and  in 
vii-i       kin  is  :  :  mi ;  linerv.  mav  be  considered  as  levers  of 

*  I-  5  -  =  ~e  principle  that,  in  ra^ng  a  window,  the  hand  ^(mlq 
v..  ^  11  idV  of  the  sa^,  as  it  will  then  be  esskj  raised ;  where - 

, ,  ^  iiearer  to  one  ade  than  the  other,  the  centre  cf 

.  will  cause  the  farther  side  to  bear  against  the 

XTa-tQc.  ana  ocis::-i::      ir^f  irLrtZ'jii. 


83.  What  is  a  lever  o:  ' 
power  exceed  the  weight 
mnipies  of  IcTeis  of  the  thiic 


nd  ?    In  what  propordon  mus;  the 
£x{4ain  Fig.  30.    Give  soom  ex- 


ME(;ilAMCS. 


59 


this  kind,  when  the  power  that  moves  tliem  ac^s  on  tlie  pinion, 
near  tlie  centre  of  motion,  and  the  resistance  to  be  overcome 
acts  on  tlie  teeth  at  the  circumference.  But  liere  the  advan- 
tage gained  is  the  change  of  slow  into  rapid  motion.  The  sails 
of  vessels  are  constructed  on  the  principle  of  the  lever/-^ 

84.  The  Pulley  is  a  small  wheel  turning  on  an  axis, 
with  a  string  or  rope  in  a  groove  running  around  it. 

There  are  two  kinds  of  pulleys — the  fixed  and  the 
moveable.  The  fixed  pulley  is  a  pulley  that  has  no 
other  motion  than  a  revolution  on  its  axis,  and  it  is  used 
only  for  changing  the  direction  of  motion. 

Fig.  31  represents  a  fixed  pulley.  P  is  a  small  wheel  turn- 
ing on  its  axis,  with  a  string  running  round  it  in  a 
groove.  W  is  a  weight  to  be  raised,  F  is  the  force  or  Fig.  3i. 
power  applied.  It  is  evident  that,  by  pulling  the 
string  at  F,  the  weight  must  rise  just  as  much  as  the 
string  is  drawn  down.  As,  therefore,  the  velocity  of 
the  weight  and  the  power  is  precisely  the  same,  it  is 
manifest  that  they  balance  each  other,  and  that  no 
mechanical  advantage  is  gain-ed.  But  this  pulley  is 
very  useful  for  changing  the  direction  of  motion.  If, 
for  instance,  we  wish  to  raise  a  weight  to  the  top  of  a 
high  building,  it  can  be  done  with  the  assistance  of  a  fixed 
pulley,  by  a  man  standing  below.f  ^  A  curtain,  or  a  sail,  also, 
can  be  raised  by  means  of  a  fixed  pulley,  without  ascending 
with  it,  by  drawing  down  a  string  running  over  the  pulley. 

85.  The  moveable  pulley  differs  from  the  fixed  pulley 

*  It  may  perhaps  assist  the  memory  to  retain  the  relative  positions  of 
the  weight  the  power  and  the  fulcrnm  in  the  three  kinds  of  levers,  if  the 
initials  be  presented  to  the  eye  as  follows: 

First  kind,       W.  F.  P. 

Second  "         F.  W.  P. 

Third    "         F.  P.  W. 
t  The  fixed  pulley  operates  on  the  same  principle  as  a  lever  of  the  first 
kind  with  equal  arms,  where  the  fulcrum  being  in  the  centre  of  gravity, 
the  power  and  the  weight  are  equally  distant  from  it,  and  no  mechanical 
advantage  is  gained. 


84.  What  is  a  pulley?  How  many  kinds  of  pulleys  are  there?  What 
are  they?  What  is  a  fixed  pulley?  Explain  Fig.  31.  What  advantage 
is  gained  by  this  pulley  ?  What  is  the  use  of  this  pulley  ?  Upon  what 
principle  does  the  fixed  pulley  operate  ? 


60 


NATURAL  PHILOSOPHY. 


by  being  attached  to  the  weight ;  it  therefore  rises  and 
falls  with  the  weight. 

Fig.  32  represents  a  moveable  pulley,  Tvitli  the 
weight  W  attached  to  it  by  a  hook  below.  One 
end^of  the  rope  is  fastened  at  F  ;  and  as  the  power 
P  draws  the  weight  upwards,  the  pulley  rises  with 
the  weight.  Now,  in  order  to  raise  the  weight  one 
inch,  it  Ts  evident  that  both  sides  of  the  string  must 
be  shortened ;  in  order  to  do  which,  the  power  P 
must  pass  over  two  inches.  As  the  velocity  of  the 
power  is  double  that  of  the  weight,  it  follows  that 
a  power  of  one  pound  will  balance  a  weight  on  the 
moveable  pulley  of  two  pounds. 

86.  The  power  gained  by  the  use  of  pulleys  is  ascer- 
tained by  multiplying  the  number  of  moveable  pulleys 
by  2.* 

1.  A  weight  of  12  pounds  may  be  balanced  by  a  power  of 
9  pounds  with  four  pulleys  ;  by  a  power  of  18  pounds  with 
two  pulleys  ;  or  by  a  power  of  36  pounds  with  one  pulley. 
But  in  each  case  the  space  passed  over  by  the  power  must  be 
double  the  space  passed  over  by  the  weight,  multiplied  by  the 
number  of  moveable  pulleys.  That  is,  to  raise  the  weight  one 
foot,  with  one  pulley,  the  power  must  pass  over  two  feet,  with 
two  pulleys  four  feet,  with  four  pulleys  eight  feet. 

2.  Fig.  33  represents  a  system  of  fixed  and  moveable  pulleys 
In  the  block  F,  there  are  four  fixed  pulleys,  and 
in  the  block  M  there  are  four  moveable  pulleys, 
all  turning  on  their  common  axis,  and  rising  and 
falling  with  the  weight  W.  The  moveable  pulleys 
are  connected  with  the  fixed  ones  by  a  string  at- 
tached to  the  hook  H,  passing  over  the  ahernate 
grooves  of  the  pulleys  in  each  block,  forming 
eight  cords,  and  terminating  at  the  power  P. 
Now  to  raise  the  weight  one  foot,  it  is  evident 
that  each  of  the  eight  cords  must  be  shortened 
one  foot,  and,  consequently,  that  the  power  P 
must  descend  eight  times  that  distance.  The 

*  This  rule  applies  only  to  the  moveable  pulleys  in  the  same  block. 


Fig.  33. 


85.  How  does  the  moveable  pulley  differ  from  the  fixed  pulley  ?  Ex- 
plain Fig.  32. 

86.  How  can  the  power  gn.ined  by  the  use  of  the  moveable  pulley  be  as- 
certained ?  What  illustration  of  this  is  given  ?  What  does  Fig.  33  represent  7 


MECHANICS.  61 

po^er,  thei-efore,  must  pass  over  eight  times  the  distance  that 
the  weiglit  moves. 

87.  Pulleys  act  on  the  same  principle  with  the  lever, 
the  deficiency  of  the  strength  of  the  power  being  com- 
pensated by  its  superior  velocity. 

Now,  as  we  cannot  increase  om-  natural  strength,  but  can 
increase  the  velocity  of  motion,  it  is  evident  that  we  are 
enabled,  by  .pulleys  and  other  mechanical  powers,  to  reduce 
the  resistance  -or  weight  of  any  body  to  the  level  of  our 
strength. 

1.  Practical  use  of  Pulleys,  Pulleys  are  used  to  raise  goods 
into  warehouses,  and  in  ships,  (fee.  to  draw  up  the  sails.  Both 
kinds  of  pulleys  are  in  these  cases  advantageously  applied  ;  for 
the  sails  are  raised  up  to  the  masts  by  the  sailors  on  dock,  by 
means  of  the  fixed  pulleys,  while  the  labor  is  facilitated  by  the 
mechanical  power  of  the  moveable  ones. 

2.  Bo'  h  fixed  and  moveable  pulleys  are  constructed  in  a  great 
variety  of  forms,  but  the  principle  on  which  all  kinds  are  coii- 
sti-uc.cd,  is  the  same.  What  is  generally  called  a  tackle  and 
fall,  or  a  block  and  tackle,  is  nothing  more  than  a  pulley. 
jPulleys  have  likewise  lately  been  attached  to  the  harness  of  a 
horse,  to  enable  the  driver  to  govern  the  animal  with  less 
exertion  of  sti  ength. 

3.  It  may  be  observed,  in  relation  to  the  mechanical  powers 
in  general,  that  power  is  always  gained  at  the  expense  of  time 
and  velocity ;  that  is,  the  same  power  which  will  raise  one 
pound  in  one  minute,  will  raise  two  pounds  in  two  minutes,  six 
pounds  in  six  minutes,  sixty  pounds  in  sixty  minutes,  &c. ;  and 
that  the  same  quantity  of  force  used  to  raise  two  pounds  one 
foot,  will  raise  one  pound  two  feet,  (fee.  And,  further,  it  may 
be  stated  that  the  product  of  the  weight,  multiplied  by  the 
velocity  of  the  weight,  will  always  be  equal  to  the  product  of 
the  power  muhiplied  by  the  velocity  of  the  power.  Hence 
w^e  have  the  following  rule.  The  power  is  in  the  same  j^ropor- 
tion  to  the  lueight  as  the  velocity  of  the  loeight  is  to  the  velocity 
of  the  povjer. 

87.  Upon  what  principle  do  pulleys  act?  What  advantage  is  gained 
by  the  use  of  pulleys  and  other  mechanical  powers  ?  What  are  some  of 
the  practical  uses  of  the  pulley?  What  is  a  tackle  and  fall?  Is  there 
any  time  or  velocity  gained  hy  the  power  in  the  mechanical  powers?  To 
what  is  the  product  of  the  weight,  nialtiplied  by  its  vebclty,  always  equal  ? 
What  rule  is  given  ? 


62 


NATURAL  PHILOSOPHY. 


88.  The  wheel  and  axle  consists  of  two  wheels  of 
different  sizes,  revolving  together  around  the  same  cen- 
tre of  motion. 

The  place  of  the  smaller  wheel  is  generally  supplied 
by  a  cylinder,*  which  forms  the  axle. 

1 .  The  wheel  and  axle,  though  made  in  many  forms,  will  easily 
be  understood  by  inspecting  Figs.  34  and  35.    In  Fig.  34,  P 

represents  the  larger  wheel, 
where  the  power  is  applied ; 
C  the  smaller  wheel  or 
cylinder,  which  is  the  axle, 
and  W  the  weight  to  be 
raised.  The  advantage  gain- 
ed is-  in  proportion  as  the 
circumference  of  the  wheel  is 
greater  than  that  of  the  axle. 
That  is,  if  the  circumference 
of  the  wheel  be  six  times 
the  circumference  of  the 
axle,  then  a  power  of  one 
pound  applied  at  the  wheel 
will  balance  a  power  of  six 
pounds  on  the  axle. 

2.  Sometimes  the  axle  is 
constructed  with  a  winch  or 
handle,  as  in  Fig.  35,  and  sometimes  the  wheel  has  project-, 
ing  spokes,  as  in  Fig.  34. 

3.  The  principle  upon  which  the  wheel  and  axle  is  con- 
structed is  the  same  with  that  of  the  other  mechanical  powers, 
the.  want  of  power  being  compensated  by  velocity.  It  is 
evident  (from  the  Figs.  34  and  35)  that  the  velocity  of  the 
circumference  of  the  wheel  is  as  much  greater  than  that  of 
the  axle  as  it  is  further  from  the  centre  of  motion ;  for  the 
wheel  describes  a  great  circle  in  the  same  time  that  the 

*  A  cylinder  is  a  long  circular  body  of  uniform  diameter,  with  ex- 
tremities forming  ^qual  and  parallel  circles. 


88  Of  what  does  the  wheel  and  axle  consist?  What  is  a  cylinder? 
What  figures  illustrate  the  wheel  and  axle?  Explain.  To  what  is  the 
advantage  gained  in  proportion  ?  What  does  Fig.  34  represent  ?  Fig.  35  ? 
Upon  what  principle  is  the  wheel  and  axle  constructed  ?  Explain  by  Figs. 
34  and  35. 


MF.CIIANICS. 


63 


axle  desci  ibes  a  small  one ; 
therefore  the  power  is  in- 
creased in  the  same  pro- 
portion as  the  circumfer- 
ence of  the  wheel  is  greater 
than  that  of  the  axle.  If 
the  velocity  of  the  wheel  be 
twelve  times  greater  than 
that  of  the  axle,  a  power 
of  one  pound  on  the  wheel 
will  support  a  weight  of 
twelve  pounds  on  the  axle. 

89.  The  wheel  and  axle  are  constructed  on  the  same 
principle  with  the  lever ;  the  axle  acting  the  part  of  the 
shorter  arm  of  the  lever,  the  wheel  that  of  the  longer 
arm. 

1.  The  capstan,'^  on  board  of  ships  and  other  vessels,  is 
constructed  on  the  principle  of  the  wheel  and  axle.  It  consists 
of  an  axle  placed  uprightly,  with  a  head  or  drum,  pierced  with 
holes  for  the  lever,  or  levers,  which  supply  the  place  of  the 
wheel. 

2.  Windmills,  lathes,  the  common  windlass,"^  used  for  draw- 
ing water  from  w^ells,  cmd  the  large  wheels  in  mills,  are  all  con- 
structed on  the  principle  of  the  wheel  and  axle. 

3.  Wheels  are  a  very  essential  part  to  most  machines ;  they 
are  applied  in  different  ways,  but  when  affixed  to  the  axle  their 
mechanical  power  is  always  in  the  same  proportion ;  that  is,  as 
the  circumference  of  the  wheel  exceeds  that  of  the  axle,  so 
much  will  the  power  be  increased.  Therefore  the  larger  the 
wheel  and  the  smaller  the  axle,  the  greater  will  be  the  power 
obtained. 

4.  Fly-wheels  are  heavy  wheels  used  to  accumulate  power 

*  The  difference  between  a  capstan  and  a  windlass  lies  only  in  the 
position  of  the  wheel.  If  the  wheel  turn  horizontally  it  is  called  a  cap- 
stan ;  if  vertically,  a  windlass. 


89.  Upon  what  principle  are  the  wheel  and  axle  constructed?  Explain 
how.  Upon  what  principle  is  the  capstan  on  board  of  vessels  constructed  ? 
Of  what  does  it  consist?  What  other  things  are  mentioned  as  constructed 
upon  this  principle?  Are  wheels  an  essential  part  to  most  machines?  Aro 
they  applied  in  more  than  one  way?  When  they  are  affixed  to  the  axle, 
in  what  proportion  is  the  power  increased? 


61 


NATURAL  PHILOSOPHY. 


and  distribute  it  equally  among  all  the  parts  of  a  machine. 

They  are  caused  to  revolve  by  a  force  applied  to  the  axle ; 
and  when  once  set  in  motion  continue  by  their  inertia  to  move 
for  a  long  time.  As  their  motion  is  steady  and  without  sudden 
jerks,  they  serve  to  steady  the  power,  and  cause  a  machine  to 
work  vnth  regularity. 

5.  Cranks  are  sometimes  connected  with  the  axle  of  a 
wheel,  either  to  give  or  to  receive  its  motion.    They  are  made 
by  bending  the  axle  in  such  a  manner  as  to  form  j,.^ 
four  right  angles  facing  in  different  directions,  as 
is  represented  in  Fig.  36.    This  is  seen  in  lathes 
and  many  other  kinds  of  machinery.    Cranks  are 
often  used  to  change  the  motion  from  rectilinear 
to  circular,  or  from  circular  to  rectilinear. 

6.  In  connexion  with  the  wheel  and  axle,  it  is  proper  to 
mention  the  subject  of  complex  wheel- work.  It  has  already 
been  stated  that  the  velocity  of  the  wheel  is  greater  than  that 
of  the  axle ;  and  that  this  velocity  is  in  proportion  to  the 
relative  size  of  the  wheel  compared  with  that  of  the  axle. 
Advantage  is  taken  of  this  circumstance  in  the  construction  of 
machinery,  by  such  an  arrangement  of  the  parts  as  will  enable 
us  to  increase  or  lessen  the  speed  at  pleasure.  For  it  is 
evident  that  if  the  power  be  applied  to  the  axle,  and  machinery 
attached  to  the  wheel,  rapid  motion  Avill  be  produced ;  and  on 
the  contrary,  if  the  power  be  apphed  to  the  wheel  and  the 
machinery  to  the  axle,  slow  motion  will  be  produced. 

v.  Fig.  37  represents  four  wheels  with  their  axles,  each 
wheel  acting  on  the  axle  of  the  adjoining 
wheel.  F  is  the  power  applied  to  the 
axle  of  the  wheel  d.  Now,  supposing  the 
circumference  of  each  wheel  to  be  six  times 
the  circumference  of  each  axle,  it  is  evident 
that  each  time  the  wheel  d  revolves  it  must 
cause  the  wheel  c  to  make  six  revolutions, 
because  the  circumference  of  the  wheel  d  is 

What  are  fly-wheels,  and  for  what  are  they  used?  How  are  they  made 
to  revolve  ?  When  once  set  in  motion,  what  causes  them  to  move  on  for 
some  time?  Of  what  service  are  they  in  a  machine?  For  what  are 
cranks  sometimes  connected  with  the  axle  of  a  wheel  ?  Hov/  are  they 
made?  What  does  Fig.  36  represent?  For  what  are  cranks  often  used? 
How  does  the  velocity  of  the  wheel  compare  with  that  of  the  axie?  To 
what  is  this  velocity  :n  proportion.?  Is  any  advantage  taken  of  this  in 
dri'^^ing  machin^-ry  where  the  speed  is  to  be  increased  or  diminished  ? 


M  KCII  A  M  K  'r?, 


05 


A  six  (inu\s  the  circuinfcMciU'c  of  llic  nxK^  of  c.  \\\  like  mMrifier 
the  circumferences  of  llie  wlieels  c  and  />,  cictin<^  respect iv(;ly 
on  the  circumferences  of  tlie  axles  of  the  adjoining  wiieel,  will 
communicate  a  velocity  six  times  tj^realer  than  their  own,  and 
Avhile  the  Avheel  d  makes  one  revolution  the  wheel  c  will 
make  six,  h  thirty-six,  and  a  tw^o  hundred  and  sixteen  revolu- 
tions. 

8.  Reversing  the  figure,  and  applying  the  power  at  S  which 
communicates  with  the  circumference  of  the  wheel  a,  it  follows 
that  a  must  perform  six  revolutions  while  /;  is  performing  one, 
thirty-six  while  c,  and  two  hundred  and  sixteen,  while  d  performs 
one  revolution.  It  will  thus  be  perceived  that  a  rapid  or  a  slow 
motion  may  be  communicated  by  various  combinations  of  the 
wdieel  and  axle. 

9.  The  usual  way  of  transmitting  the  action  of  the  axles  to 
the  adjoining  wheels  is  by  means  of  teeth  or  cogs,  raised  on 
their  surfaces.  The  cogs  on  the  surface  of  the  wheels  are 
generally  called  teeth,  and  those  on  the  surface  of  the  axle  are 
called  leaves.  The  axle  itself,  when  furnished  with  leaves,  is 
called  a  pinion. 

10.  Fig.  38  represents 
a  connexion  of  coofo^ed 
wheels.  The  wheel  B 
being  moved  by  a  string 
around  its  circumference 
is  a  simple  wheel  with- 
out teeth.  Its  axle  being 
furnished  with   coo^s  or 

o 

leaves,  to  which  the  teeth 
of  the  wheel  D  are  fitted, 
communicates  its  motion 
to  D,  which,  in  like  man- 
ner, moves  the  wheel  G. 
The  power  P  and  the  weight  W  must  be  attached  to  the  cir- 
cumference of  the  v/heel  or  of  the  axle  according  as  a  slow  or 
a  rapid  motion  is  desired. 

11.  V/heels  are  sometimes  turned  by  bands,  as  in  Fig.  39  ; 


How  would  rapid  motion  be  produced?  Slow  motion?  Explain  Fig. 
37.  What  is  the  usual  way  of  transmitting  the  action  of  the  axles  to  the 
adjoining  wheels  ?  What  are  the  cogs  on  the  surface  of  the  wheel  called  ? 
Those  on  the  axle?  What  is  a  pinion?  Explain  Fig.  38.  By  what  are 
wheels  sometimes  turned  ? 


Fig.  38. 

D  C 


66 


NATURAL  PHILOSOPHY. 


and  the  motion  communiccited  may  be  direct 
or  reversed  by  attaching  the  band  as  repre- 
sented in  Figs.  39  and  40.  When  the  wheel 
and  the  axle  from  which  it  receives  motion  are 
intended  to  revolve  in  the  same  direction,  the 
strap  is  not  crossed,  but  is  applied  as  in  Fig. 
39.  But  when  the  wheel  is  to  revolve  in  a 
direction  contrary  to  the  revoltition  of  the 
axle,  the  strap  is  crossed,  as  in  Fig.  40. 

Different  directions  may  be  given  to  the  motion  pro- 


Fig.  42. 


Fig.  40. 


duced  by  wheels,  by  varying  the  position  of  their  axles,  and 
causing  them  to  revolve  in  different  planes,  as  in  Fig.  41  ;  or 
by  alteiing  the  shape  and  position  of  the  cogs,  as  in  Fig  42. 

90.  The  inclined  plane  consists  of  a  plain  surface  in- 
clined to  the  horizon,  and  the  advantage  gained  by  the 
inclined  plane  is  in  proportion  as  the  length  of  the  plane 
exceeds  its  perpendicular  height. 

1.  Fig.  43  represents  an  inchned  plane, 
its  length,  and  W  a.. weight  which  is  to 
be  moved  on  it.  If  the  length  C  B  be 
four  times  the  height  C  A,  then  a  power 
of  one  pound  at  C  will  balance  a  weio'ht 
of  four  pounds  on  the  inchned  plane 
C  B. 


C  A  its  heiirht,  C  B 


What  figure  represents  one  ?  In  what  way  can  the  motion  be  made 
direct  or  reversed?  What  does  Fig.  39  represent  ?  Fig.  40  ?  In  what 
way  can  different  directions  be  given  to  the  motion  produced  by  wheels  ? 
What  dots  Fig.  41  represent  ?    Fig.  42  ? 

90.  What  is  an  inclined  plane  ?  AVhat  figure  represents  an  inclined 
plane  ?  Explain  the  figure.  To  what  is  the  advantage  gained  by  the 
use  of  the  inclined  plane  in  proportion? 


MECHANICS. 


67 


2.  The  greater  the  indinatioii  of  tlie  plane,  the  greater  must 
be  its  po'pendicular  height,  compared  with  its  length ;  and,  of 
course,  the  greater  must  be  the  power  to  elevate  a  weight 
ah.)ng  its  surface. 

3.  Instances  of  the  appUcation  of  the  inclined  plane  are  vei  y 
common.  Sloping  planks  or  pieces  of  timber  leading  into  a 
cellar,  and  on  which  casks  are  rolled  up  and  down ;  a  plank  or 
bo.trd  with  one  end  elevated  on  a  step,  for  the  convenience  of 
trundling  wheelbarrows,  or  rolling  barrels  mto  a  store,  &c., 
are  inclined  planes. 

4.  The  advantage  gained  by  the  use  of  the  inclined  plane, 
like  that  of  the  other  mechanical  powers,  is  attended  by  a  loss 
of  time ;  for  the  weight,  instead  of  moving  directly  up  the 
ascent,  must  move  the  whole  length  of  the  plane. 

5.  Chisels  and  other  cuttinof  instruments,  which  are  ckam- 
ferecl  or  sloped  only  on  one  side,  are  constructed  on  the  prm- 
ciple  of  the  inclined  plane. 

91.  The  wedge  consists  of  two  inclined  planes  united 
at  their  bases  ;  and  the  advantage  gained  by  the  use  of 
the  wedge  is  in  proportion  as  the  length  exceeds  one 
half  the  thickness  of  its  converging  sides. 

1.  Fig.  44  represents  a  wedge.  The  line  a  h  repre- 
sents  the  base  of  each  of  the  inclined  planes  of  which 
it  is  composed,  and  at  which  they  are  united. 

2.  The  wedge  is  a  very  important  mechanical 
power,  used  to  split  rocks,  timber,  &c.,  which  could 
not  be  effected  by  any  other  power. 

3.  Axes,  hatchets,  knives,  and  all  other  cutting  in- 
struments chamfered,  or  sloped  on  both  sides,  are  constructed 
on  the  principle  of  the  wedge. 

92.  The  screw  consists  of  an  inclined  plane,  wound 
round  a  cylinder,  thus  producing  a  circular  inclined 


What  follows  from  the  greater  or  less  inclmation  of  the  plane?  Give 
some  instances  of  the  application  of  the  inclined  plane.  Is  any  time  gained 
by  the  use  of  the  inclined  plane?  Upon  what  principle  are  chisels  and 
other  cutting  instruments,  which  are  sloped  only  on  one  side,  constructed  ? 

91.  Of  what  does  the  wedge  consist?  What  does  Fig.  44  represent? 
To  what  is  the  advantage  gained  by  the  wedge  in  proportion  ?  Of  what 
use  is  the  wedge  ?    Give  some  examples  of  the  wedge. 

92.  Of  what  does  the  screw  consist  ? 


68 


NATURAL  PHILOSOPHY. 


plane,  and  forming  what  is  called  the  threads  of  the 
screw.  .  . 

The  advantage  gained  in  the  use  of  the  screw  is  m 
proportion  as  th'e  circumference  described  by  the  handle 
exceeds  the  distance  between  the  threads  of  the  screw. 

The  screw  is  generally  composed  of  two  parts,  the 
screw  and  the  nut ;  or,  as  they  are  generally  called,  the 
convex  and  concave  screw\ 

1.  Fio-.  45  represents  the  screw  and  the  nut.  S  is  the  con- 
vex  screw,  (which  is  an  inclined  plane  wound 
round  a  cvlinder,)  jS"  is  the  nut,  or  concave  screw, 
which  has  a  spiral  groove,  to  which  the  thread 
of  the  convex  screw  is  accurately  fitted.  L  is  a 
lever  attached  to  the  nut,  to  which  the  power  is 
applied.  By  turning  the  lever 
in  one  direction  the  nut  ascends, 
and  by  turning  it  in  the  opposite 

direction,  the  nut  descends  on  the  screw.^  In 
this  fio'ure  the  screw  is  fixed,  and  the  nut  is 
moveable. 

2.  Fig.  46  represents  another  screw,  which 

is  moveable.  The  nut  is  fixed  to  the  frame,  and        '  ' 

the  screw  ascends  or  descends  as  the  lever  L  is  turned. 

*  Although  the  screw  is  mentioned  as  one  of  the  six  mechanical  powers, 
it  is,  in  reality  a  compound  power,  consisting  of  a  lever  and  an  inclined 
plane.  The  power  of  the  screw  being  estimated  by  the  distance  of  the 
threads,  the  closer  the  threads  the  greater  is  the  power ;  but  here,  again, 
the  increase  of  power  is  procured  by  an  increase  of  velocity,  for  a  loss  of 
time.  For  if  the  threads  be  a  quarter  of  an  inch  apart,  the  power  must 
move  through  the  wlioie  circumference  of  the  circle  described  by  the 
lever,  in  order  to  move  the  resistance  a  quarter  of  an  inch.  The  screw, 
with  its  appendage  the  lever,  is  therefore  used  for  the  purpose  of  moving 
large  or  heavy  bodies  through  small  distances.  Its  power  may  be  increased 
by  lengthening  the  lever.  The  screw  is  applied  to  presses  of  all  kinds 
where  great  power  is  required,  such  as  bookbinders'  presses,  cider  and 
wine  presses,  &c. 

Of  how  many  parts  is  it  generally  composed  ?  What  are  they  ?  What 
figure  represents  the  screw  and  the  nut  ?  Explain  the  figure.  How  does 
Fig.  45  difFsr  from  Fig.  46  ?  Note.  Is  the  screw  a  simple  or  compound 
power?  How  is  the  power  of  the  screw  estimated  ?  How  does  the  close, 
ness  of  the  thread  afFect  the  power?  What  is  the  use  of  the  screw  ?  How 
can  its  power  be  increased  ?    To  what  is  the  screw  appUed  1 


A\ — ^  A. 


MECHANICS. 


69 


3.  By  friction  in  macliinery  is  meant  the  resistance  which 
bodies  meet  with  in  rubbing  against  each  other. 

4.  There  are  two  kinds  of  friction,  the  rolhng  and  the 
sliding.  The  rolHng'  fiiction  is  caused  by  the  rolhng  of  a 
circular  body.  The  sliding  friction  is  produced  by  the  shding 
or  dragging  of  a  body  over  a  flat  surface.  The  sliding  friction 
is  overcome  with  more  difficulty  than  the  rolling.  In  calcula- 
ting the  power  of  a  machine,  an  allowance  must  always  be 
made  for  friction.  It  is  usually  computed  that  friction  de- 
stroys one-third  of  the  power  of  a  machine.^ 

5.  Friction  is  caused  by  the  unevenness  of  the  surfaces 
which  come  into  contact  ;f  and  it  is  diminished  in  proportion 
as  the  surfaces  are  smooth  and  well  pohshed.  Oil,  grease, 
black-lead,  or  powdered  soap-stone,  is  used  to  lessen  faction, 

*  When  finely  polished  iron  is  made  to  rub  on  bell  metal,  the  friction 
is  said  to  be  reduced  to  about  one-eighth.  Mr.  Babbit  of  Boston  has  pre- 
pared a  composition  for  the  wheel-boxes  of  locomotive  engines  and  other 
machinery,  which  it  is  said  has  still  further  reduced  the  amount  of  friction. 
This  composition  is  now  much  in  use.  As  the  friction  between  rolling  bodies 
is  much  less  than  in  those  that  drag,  the  axle  of  large  wheels  is  sometimes 
made  to  move  on  small  wheels  or  rollers.  These  are  called  friction  wheels, 
or  friction  rollers.  They  turn  round  their  own  centre  as  the  wheel  con- 
tinues its  motion. 

t  All  bodies,  how  well  soever  they  may  be  polished,  have  inequalities  in 
their  surfaces,  which  may  be  perceived  by  a  microscope.  When,  there- 
fore, the  surfaces  of  two  bodies  come  into  contact,  the  prominent  parts  of 
the  one  will  often  fall  into  the  hollow  parts  of  the  other,  and  cause  more 
or  less  resistance  to  motion.  Friction  increases,  1st,  as  the  weight  or  pres- 
sure is  increased  ;  2d,  as  the  areas  of  the  surfaces  in  contact  are  increased  ; 
3d,  as  the  roughness  of  the  surface  is  increased.  Friction  may  be  diminished, 
1st,  by  lessening  the  w^eight  of  the  body  in  motion  ;  2d,  by  mechanically 
reducing  the  asperities' of  the  sliding  surfaces  ;  3d,  by  lessening  the  amount 
of  surface  of  homogeneous  bodies  in  contact  with  each  other  ;  4th,  by  con- 
verting a  sliding  mto  a  rolling  motion  ;  5th,  by  applying  some  suitable 
unguent. 

What  is  meant  by  friction  in  machinery?  How  many  kinds  of  friction 
are  there  ?  What  are  they  ?  How  is  the  rolling  friction  produced  ?  The 
sliding?  Which  is  overcome  with  the  less  difficulty,  the  rolling  or  sliding? 
What  allowance  must  always  be  made,  in  calculating  the  power  of  a  ma- 
chine? What  proportion  of  the  power  is  usually  computed  to  be  destroyed 
by  friction?  Between  which  is  friction  the  less,  rolling  bodies  or  those 
that  slide?  What  causes  friction  ?  In  what  proportion  is  it  diminished? 
In  what  manner  can  it  be  lessened  ? 


70 


NATURAL  THILOSOPHY. 


because  they  act  as  a  polish  by  fiUing  up  the  cavities  of  the 
rubbing  surfaces,  and  thus  making  them  shde  more  easily  over 
each  other. 

6.  From  what  has  been  stated  with  regard  to  the  mechanical 
powers,  it  appears  that  by  their  aid  a  man  is  enabled  to  per- 
form works  to  which  his  unassisted  natural  strength  is  wholly 
inadequate.  But  the  power  of  all  machines  is  hmited  by  the 
strength  of  the  materials  of  which  they  are  composed.  Iron, 
which  is  the  strongest  of  all  substances,  will  not  resist  a  strain 
beyond  a  certain  limit.  Its  cohesive  attraction  may  be  de- 
stroyed, and  it  can  Avithstand  no  resistance  which  is  stronger 
than  its  cohesive  attraction.  Besides  the  strength  of  the  ma- 
terials, it  is  necessary,  also,  to  consider  the  time  which  is  ex- 
pended in  the  apphcation  of  mechanical  assistance.  Archimedes 
is  said  to  have  boasted  to  Hiero,  king  of  Syracuse,  that,  if  he 
would  give  him  a  place  to  stand  upon,  he  would  move  the 
whole  world.  In  order  to  do  this,  Archimedes  must  himself 
have  moved  over  as  much  more  space  than  he  moved  the 
world,  as  the  weight  of  the  world  exceeded  his  own  weight ; 
and  it  has  been  computed  that  he  must  have  moved  with  the 
velocity  of  a  cannon  ball  for  a  milhon  of  years,  in  order  to 
move  the  earth  the  twenty-seven  milhonth  part  of  an  inch. 

7.  Wheels  are  used  on  vehicles  to  diminish  the  friction  of 
the  road.  The  larger  the  cu*cumference  of  the  wheel,  the 
more  readily  it  will  overcome  any  obstacles,  such  as  stones,  or 
inequahties  in  the  road.'^ 

93.  A  mediumf  is  the  substance,  solid  or  fluid,  which 
surrounds  a  body. 

*  In  descending  a  steep  hill,  the  wheels  of  a  carriage  are  often  locked, 
(as  it  is  called.)  that  is,  fastened  in  such  a  manner  as  to  prevent  their 
turning ;  and  thus  the  rolling  is  converted  into  the  sUding  friction,  and  the 
vehicle  descends  more  safely. 

Castors  are  put  on  the  legs  of  tables  and  other  articles  of  furniture,  to 
facilitate  the  moving  of  them ;  and  thus  the  sliding  is  converted  into  the 
rolling  friction. 

t  The  plural  of  this  word  is  media. 

What  is  the  use  of  wheels  ?  In  what  proportion  do  they  overcome  the 
obstacles,  such  as  stones,  &c.,  in  the  road?  Why,  in  descending  a  steep 
hill,  are  the  wheels  of  a  carriage  often  locked  ?  How  do  castors,  which 
are  put  upon  furniture,  facilitate  the  moving  of  it  ?  How  is  the  motion  of 
all  bodies  influenced? 

93.  What  is  meant  by  a  medium  1    Give  examples. 


MECHANICS. 


71 


Thus,  air  is  the  medium  which  surrounds  a  bird  when  flying ; 
water  is  the  medium  which  surrounds  the  lish  when  swim- 
ming, &c. 

94.  The  motion  of  all  bodies  is  influenced  by  the 
medium  in  which  they  move  ;  and  the  resistance  of  a 
medium  is  in  exact  proportion  to  its  density. 

A  body  falhng  through  water  meets  with  more  resistance 
than  when  falhng  through  air,  because  water  is  a  denser 
medium  than  air.  If  a  machine  could  be  worked  in  vacuo, 
(that  is,  in  a  vacuum,  or  a  space  where  there  is  neither  air  nor 
any  thing  else  to  impede  it,)  and  without  friction,  it  would  be 
perfect. 

95.  The  main-spring  of  a  watch  consists  of  a  long 
ribbon  of  steel,  closely  coiled,  and  contained  in  a  round 
box.  It  is  employed  instead  of  a  weight,  to  keep  up  the 
motion. 

1.  As  the  spring,  when  closely  coiled,  exerts  a  stronger  force 
than  when  it  is  partly  loosened,  in  order  to  correct  this  in- 
equality, the  chain  through  which  it  acts  is  wound  upon  an 
axis  surrounded  by  a  spiral  groove,  (called  fusee?)  gradually 
increasing-  in  diameter  from  the  top  to  the  bottom;  so  that,  in 
proportion  as  the  strength  of  the  spring  is  diminished,  it  may 
act  on  a  larger  lever,  or  a  larger  wheel  and  axis. 

2.  Fig.  47  represents  a 

spring  coiled  in  a  round  Fig.  47.  A 

box.   A  B  is  the  fusee,  sur-  i  .1 

rounded  by  a  spiral  groove, 
on  which  the  chain  C  is 
wound.  When  the  watch  is 
recently  wound,  the  spring 
is  in  the  greatest  state  of 
tension,  and  will,  therefore, 
turn  the  fusee  by  the  smallest  groove,  on  the  prmciple  of  the 
wheel  and  axle.  As  the  spring  loses  its  force  by  being  partly 
unwound,  it  acts  upon  the  larger  circles  of  the  fusee ;  and  the 

94.  To  what  is  the  resistance  of  a  medium  in  proportion  ?  What  ilhis- 
tration  is  given  ?    When  would  a  machine  be  perfect  ? 

95.  Of  what  does  the  main-spring  of  a  watch  consist  ?  What  is  its  use  ? 
Does  the  spring  exert  a  stronger  force  when  closely  coiled,  or  when  partly 
loosened  ?  What  is  done  in  order  to  correct  this  inequality  ?  What  does 
Fig.  47  represent  ?  Explain. 


72 


NATURAL  PHILOSOPHY. 


want  of  strength  in  the  spring  is  compensated  by  the  me- 
clianical  aid  of  a  larger  wheel  and  axle  in  the  larger  grooves. 
By  this  means  the  spring  is  made  at  all  times  to  exert  an  equal 
power  upon  the  fusee.  The  motion  is.  communicated  from 
the  fusee  by  a  cogged  wheel  which  turns  with  the  fusee. 

96.  The  name  of  governor  has  been  given  to  an  in- 
genious piece  of  mechanism,  invented  by  Mr.  James 
Watt,  which  is  used  to  regulate  the  supply  of  power  in 
machinery ;  as  that  of  steam  in  steam-engines,  and  of 
water  in  water-mills. 

Fig.  48  represents  a  governor.  A  B 
and  A  C  are  two  levers  or  arms,  loaded 
with  heavy  balls  at  their  extremities 
B  and  C,  and  suspended  by  a  joint  at 
A  upon  the  extremity  of  a  revolving 
shaft,  AD.  At  a  is  a  collar,  or  sliding 
box,  connected  with  the  levers  by  the 
rods  h  a  and  c  a,  with  joints  at  their 
extremities.  When  the  shaft  A  D  re- 
volves rapidly,  the  centrifugal  force  of 
the  balls  B  and  C  will  cause  them  to 
diverge  in  their  attempt  to  fly  off",  and 
thus  raise  the  collar  a,  by  means  of 
the  rods  h  a  and  c  a.  On  the  contrary, 
when  the  shaft  A  D  revolves  slowly, 

the  weights  B  and  C  will  fall  by  their  own  weight,  and  the 
rods  h  a  and  c  a  wih  cause  the  collar  a  to  descend.  The  steam- 
valve  in  a  steam-engine,  or  the  sluice-gate  of  a  water-wheel, 
being  connected  with  the  collar  a,  the  supply  of  steam  or 
water,  which  puts  the  works  in  motion,  is  thus  regulated.'-^ 

*  In  manufactures,  there  is  one  certain  and  determinate  velocity  with 
which  the  machinery  should  be  moved,  and  wliich,  if  increased  or  diminish- 
ed, would  render  the  machine  unfit  to  perform  the  work  it  is  designed  to 
execute.  Now,  it  frequently  happens  that  the  resistance  is  increased  or 
diminished  by  some  of  the  machines  which  are  worked,  being  stopped,  or 
others  put  on.  The  moving  power,  having  this  alteration  in  the  resistance, 
would  impart  a  greater  or  less  velocity  to  the  machinery,  were  it  not  for 
the  regulating  power  of  the  governor,  which  increases  or  diminishes  the 
supply  of  water  or  of  steam,  which  is  the  moving  power. 


96.  What  is  a  governor?  Explain  Fig.  48.  What  is  said  in  the  note 
of  the  use  of  the  governor  1 


HYDROSTATICS. 


73 


97.  The  knee-jomt,  or,  as  it  is  sometimes  called,  the 
toggle-pmt,  consists  of  two  rods  or  bars  connected  by  a 
joint,  and  increasing  rapidly  in  power  as  the  two  rods 
approach  to  the  direction  of  a  straight  line. 

1.  Fig.  49  represents  a  toggle-joint.    A  C 
and  B  C  are  the  two  rods  connected  by  a  joint 
C.    A  moving  force  applied  in  the  direction 
C  D  acts  with  great  and  constantly  increasing     D  - 
power  to  separate  the  parts  A  and  B. 

2.  The  operation  of  the  toggle-joint  is  seen 
in  the  iron  joints  which  are  used  to  uphold  the 

tops  of  chaises.  It  is  also  used  in  various  kinds  of  printing- 
presses,  to  obtain  the  greatest  power  at  the  moment  of  im- 
pression. 


CHAPTER  Y. 

HYDROSTATICS.* 

98.  Hydrostatics  treats  of  the  nature,  gravity,  and 
pressure  of  fluids. 

99.  A  fluid  is  a  substance  which  yields  to  the  slightest 
pressure,  and  the  particles  of  which,  having  but  a  slight 
degree  of  cohesion,  move  easily  among  themselves. 

100.  A  liquid  differs  from  a  fluid  in  its  degree  of  com- 
pressibilityf  and  elasticity. 

*  Hydrostatics  treats  of  the  properties  of  fluids  at  rest;  Hydraulics 
treats  of  fluids  in  motion. 

t  The  experiments  made  at  Florence  many  years  ago  seem  to  prove 
that  some  kinds  of  hquids — water,  for  instance — are  wholly  incompressible 
Later  experiments,  particularly  those  of  Mr.  Jacob  Perkins,  of  Newbury- 

97.  Of  what  does  the  knee-joint,  or  toggle-joint,  consist?  In  what  pro- 
portion does  it  increase  in  power  ?  What  does  Fig.  49  represent  ?  Ex- 
plain the  figure.  Give  an  instance  of  the  operation  of  the  toggle-joint. 
What  is  its  use  in  printing-presses? 

98.  Of  what  does  Hydrostatics  treat  ? 

99.  What  is  a  fluid  ?  Does  the  attraction  of  cohesion  have  much  in- 
fluence on  the  particles  of  fluids  ?    What  follows  from  this  ? 

100.  How  do  fluids  differ  from  hquids?    Can  water  be  compressed; 

4 


74 


NATURAL  PHILOSOPHY. 


1.  Tne  panicles  of  fluids  gra^-iiate  among  themselves  in  a 
more  perfect  manner  than  solids ;  because  the  strong  cohesion 
of  the  particles  of  sohd  bodies  in  some  measm-e  coimteracts 
the  effecis  of  gravity. 

2.  From  the  shght  degree  of  cohesion  in  the  particles  of 
fluids,  it  is  inferred  that  they  must  be  small,  smooth,  and 
o'lobular  ;  smooth,  because  there  appears  to  be  no  fiiction 
among  them ;  and  globular,  because  their  touching  each  other 
but  bv  a  point  will  account  for  the  shghtness  of  their  cohesion. 

3.  Fluids  cannot  be  fonned  into  ^figures,  or  preserved  in 
heaps,  on  account  of  their  want  of  cohesion. 

4.  Fluids  are  subjected  to  a  kind  of  attraction  called  capil- 
lary* attraction,  bv  which  they  are  raised  above  their  levels  in 
capillary  tubes,  or  tubes  the  'bores  of  which  are  exceedingly 
small.  '  Thus,  if  a  small  glass  tube  be  placed  in  water,  the 
water  on  the  inside  will  lise  above  the  level  of  that  on  the 
outside  of  the  tube. 

5.  The  cause  of  this  seems  to  be  nothing  more  than  the  or- 
dinary attraction  of  the  particles  of  matter  for  each  other.  The 
sides 'of  a  small  oiifice  are  so  near  to  each  other  as  to  attract 
the  panicles  of  the  fluid  on  then-  opposite  sides ;  and  as  all 
attraction  is  strongest  in  the  direction  of  the  greatest  quantity 
of  matter,  the  water  is  raised  upwards,  or  in  the  direction  of 

port,  (now  in  London,)  have  proved  that  water  is  capable  of  a  considerable 
degree  of  compression.  Fluids,  in  general,  have  a  voluntary-  tendency  to 
esjand  when  at  liberty:  but  hquids  will  not  expand  without  a  change  of 
temperature.  Heat  is  supposed  to  be  the  primary  cause  of  the  fluid  form 
of  bodies.  It  insinuates  itself  between  the  particles  of  bodies,  and  forces 
theni  asunder.  Thus,  for  instance,  ice,  without  heat,  is  a  solid :  with  heat 
it  becomes  water,  and,  with  a  greater  degree  of  heat,  it  expands  mto  an 
elastic  fluid,  called  steam. 

*  The  word  capillary  is  derived  from  the  Latin  word  capiUa,  (hair,) 
and  it  is  applied  to  this  kind  of  attraction,  because  it  is  exhibited  most 
prominently  m  tubes,  the  bores  of  which  are  as  fine  as  a  hair,  and  hence 
called  capillar)'  tubes. 

VThdit  is  supposed  to  be  the  primary  cause  of  the  fluid  form  of  bodies  ? 
^^llat  eflect  has  heat  upon  bodies  ?  What  illustration  is  given  ?  ^^'hy  do 
fluids  oravitate  in  a  more  perfect  manner  than  sohds  ?  What  is  inferred 
from  the  slight  decree  of  cohesion  in  the  particles  of  fluids  ?  Why  smooth  ? 
^Mly  globular  ?  Why  cannot  fluids  be  formed  mto  flgures,  or  preserved 
m  heaps  ?  ^^'hat  do'  you  understand  by.capillary  attraction  ?  Explain 
\he  reason  of  it. 


HYDROSTATICS.  75 

the  length  of  tlie  tube.  On  the  outside  of  tlie  tube,  the 
opposite  surfaces  cannot  act  on  the  same  cokimn  of  water 
and  tliei-elore  the  influence  of  attraction  is  here  imperceptible 
m  raising  the  fluid. 

6.  All  porous  substances,  such  as  sponge,  bread,  hnen 
sugar,  &c  may  be  considered  as  collections  of  capillary 
tubes ;  and,  for  this  reason,  water  and  other  liquids  will  rise 
in  them,  when  they  are  partly  immersed. 

1.  It  is  on  the  same  principle  that  the  wick  of  a  lamp  will 
carry  up  the  oil  to  supply  the  flame,  although  the  flame  is 
several  inches  above  the  level  of  the  oil.*  If  the  end  of  a 
towel  happen  to  be  left  in  a  basin  of  water,  it  will  empty  the 
basin  of  its  contents.  On  the  same  principle,  when  a  dry 
wedge  of  wood  is  driven  into  the  crevice  of  a  rock,  as  the 
ram  falls  upon  it,  it  will  absorb  the  water,  swell,  and  some- 
times split  the  rock.  In  this  manner,  mill-stone  quarries  are 
worked  m  Germany. 

8.  A  beautiful  experiment,  dependent  on  the  same  principle 
ot  capillary  attraction,  may  be  thus  performed.    Take  two 
*  The  reason  why  well-filled  lamps  will  sometimes  fail  to  give  light,  is. 
that  the  wick  is  too  large  for  its  tube,  and  being  thus  compressed,  the  cap- 
illary attraction  is  impeded  by  the  compression.    The  remedy  is  to  reduce 
the  size  of  the  wick.  Another  cause,  also,  that  prevents  a  clear  light,  is,  that 
the  flame  is  too  far  from  the  surface  of  the  oil.   As  capillary  attraction  acts 
only  at  short  distances,  the  surface  of  the  oil  should  always  be  within  a  short 
distance  of  the  flame.    But  another  reason  which  requires  particular  atten- 
tion,  IS,  that  all  kinds  of  oil  usually  employed  for  lamps  contain  a  glutinous 
matter,  of  which  no  treatment  can  wholly  divest  them.  This  matter  fills  the 
pores  or  capillary  tubes  of  the  wick,  and  prevents  the  ascent  of  the  oil  to  feed 
the  flame.    For  this  reason  the  wicks  of  lamps  should  be  often  renewed. 
A  wick  that  has  been  long  standing  in  a  lamp,  will  rarely  afford  a  clear 
and  bright  light.    Another  thing  to  be  noticed  by  those  who  wish  the 
lamp  to  perform  its  duty  in  the  best  possible  manner,  is,  that  the  wick  be 
not  of  such  size  as,  by  its  length  as  well  as  its  thickness,  to  fill  the  cup,  and 
thereby  leave  no  room  for  the  oil.    It  must  also  be  remembered,  that  al- 
though the  wick  when  first  adjusted  may  be  of  the  proper,  size,  the  glu- 
tinous  matter  of  the  oil,  filling  its  capillary  tubes,  causes  the  wick  to  swell, 
and  thereby  become  too  large  for  the  tube,  producing  the  same  difficulty 
as  has  already  ^Dcen  noticed  in  cases  where  the  wick  is  too  large  to  allow 
th(i  free  operation  of  capillary  attraction 

Explain  why  the  same  takes  place  in  all  porous  substances.  Explain 
ah  the  circumstances  attending  the  burning  of  a  lamp.  Explain  the  ex. 
peiiment  with  the  glass  plates. 


■yg  NATURAL  PHILOSOPHY. 

nieces  of  flat  glass,  ioined  together  at  one  side,  and  separated 
Ke  other  by  a  th>n  strip  of  wood,  card,  or  other  substance. 
When  thus  prepared,  immerse  the  glass  m  colored  water, 
having  previously  wet  the  inner  surface.  The  water  wdl  then 
rtrbetween  the  pieces  of  glass,  forming  a  beau  iful  curve 
the  higher  part  appearing  where  the  pieces  of  glass  are  m 
contact.  This  is  exemphfied  by  the  glass  plates  m  the  Bos- 
ton School  Set." 

101  The  level,  or  equilibrium  of  fluids,  is  the  tendency 
of  the  particles  so  to  arrange  themselves  that  every 
part  of  the  surface  shall  be  equally  distant  from  the 
Sntre  of  the  earth;  that  is,  from  the  point  to  which 
gravity  tends.  .        .  . 

All  fluids  have  a  tendency  to  preserve  this  equilibrium 
Hetce  the  surface  of  all  fluids,  when  in  a  state  of  res  mus 
Dartake  of  the  spherical  form  of  the  earth.  This  level  or 
Shbrium  of  flu  ds  is  the  natural  result  of  the  mdependent 
;Satlon  of  each  particle.  The  particles  of  a  jhd  body 
heina-  united  by  cohesive  attraction,  if  any  one  ot  them  be 
XmS  itw  ll  uphold  those  also  with  which  it  is  united^ 
But  when  any  particle  of  a  fluid  is  unsupported  it  is  attracted 
Sown  o  Uie  ll^el  of  the  surface  of  the  fluid  ;  and  the  readiness 
with  which  fluids  yield  to  the  slightest  ^^£^.^^5 
particle  by  its  own  weight,  to  penetrate  the  surface  of  the  flmd 
and  mix  with  it.  . 

102.  Fluids  of  different  densities  all  preserve  their 

own  equiUbrium.  ,   .    ^      *  •  +  fi,^ 

If  a  quantity  of  mercury,  water,  oil  and  air  be  put  into  the 
same  vessel,  they  will  arrange  themselves  m  the  orde^^^^^^^^ 
snecific  gravities.  The  mercury  will  sink  to  the  bottom,  tue 
Sr  win  stand  above  the  Kiercury,  the  od  f  tl-  * 
and  the  air  above  the  oil;  and  the  surface  of  each  flmd  wi 
partake  of  the  spherical  form  of  the  earth,  to  which  they  all 
respectively  gravitate. 

103.  A  vfater-level  is  an  instrument  constructed  on  the 

'  101.  What  is  meant  by  the  level  or  equilibrium  of  fluids?    Have  all 
flu  ds  a  tendency  to  preserve  this  equilibrium  ?    What  follows  from  t^s 
Of  what  is  this  level  or  equilibrium  of  fluids  the  natural  result?   How  does 
the  cavitation  of  solid  bodies  difier  from  that  of  fluids  ?  .  , 

102  Do  lids  of  different  densities  all  preserve  their  own  eqmhbnuml 
What  illustration  is  given  to  prove  this? 


HYDROSTATICS. 


77 


principle  of  the  equilibrium  of  fluids.  It  consists  of  a 
glass  tube,  partly  tilled  with  water,  and  closed  at  both 
ends.  When  the  tube  is  not  perfectly  horizontal, — 
that  is,  if  one  end  of  the  tube  be  lower  than  the  other, — 
the  water  will  run  to  the  lower  end.  By  this  means  the 
level  of  any  line  to  which  the  instrument  is  applied  may 
be  ascertained. 

Fig.  50  represents  a  water-level.    A  B  i^ig-  ^o. 

is  a  glass  tube  partly  filled  with  water.    C  a    c  b 

is  a  bubble  of  air  occupying  the  space  not  d  ^  ^  ^  b 
filled  by  the  water.  When  both  ends  of 
the  tube  are  on  a  level,  the  air-bubble  will  remain  in  the  cen- 
tre of  the  tube ;  but  if  either  end  of  the  tube  be  depressed, 
the  water  will  descend  and  the  air-bubble  will  rise.  The  glass 
tube  when  used  is  generally  set  in  a  wooden  or  a  brass  box. 
It  is  an  instrument  much  used  by  carpenters,  masons,  sur- 
veyors, &c. 

104.  Solid  bodies  gravitate  in  masses,  their  parts 
being  so  connected  as  to  form  a  whole,  and  their  weight 
may  be  regarded  as  concentrated  in  a  point  called  the 
centre  of  gravity  ;  while  each  particle  of  a  fluid  may  be 
considered  as  a  separate  mass,  gravitating  independently. 

It  is  for  this  reason  that  a  body  of  water,  in  falling,  does 
less  injury  than  a  solid  body  of  the  same  weight.  But  if  the 
v/ater  be  converted  into  ice,  the  particles  losing  their  fluid 
form,  and  being  united  by  cohesive  attraction,  gravitate  united- 
ly in  one  mass. 

105.  Fluids  not  only  press  downwards  like  sohds,  but 
also  upwards,  sidewise,*  and  in  every  direction. 

*  If  the  particles  of  fluids  were  arranged  in 
regular  columns  as  in  Fig.  51,  there  would  be 
no  lateral  pressure  ;  for  when  one  particle  is 
oerpendicularly  above  the  other,  it  can  press 
only  downwards.    But  if  the  particles  be  ar- 


103.  Upon  what  principle  is  a  water-level  constructed?  Of  what  does 
it  consist?  For  what  is  it  used?  What  figure  represents  a  water-level? 
Explain  the  figure. 

104.  In  what  manner  do  solid  bodies  gravitate?  What  is  the  centre  of 
gravity?    What  effect  has  gravity  on  the  particles  of  fluids? 

105.  How  is  the  pressure  of  fluids  exerted  ? 


Fig.  51. 


78  NATURAL  PHILOSOPHY. 

So  long  as  the  equality  of  pressure  is  undisturbed, 
every  particle  will  remain  at  rest.  If  the  fluid  be  dis- 
turbed by  agitating  it,  the  equality  of  pressure  will  be 
disturbed,  and  the  fluid  will  not  rest  until  the  equilibrium 
is  restored. 

1  The  downward  pressure  of  fluids  is  shown  by  making  an 
aperture  in  the  bottom  of  a  vessel  of  water.  Every  particle 
of  the  fluid  above  the  aperture  will  run  downwards  through 

the  opening.  , .  ^  „, 

2  The  lateral  pressure  is  shown  by  making  the  aperture  at 
the  side  of  the  vessel.  The  fluid  will  then  escape  through  the 
aperture  at  the  side.  , 

3  The  upward  pressure  is  shown  by  takmg  a  giass  tube, 
open  at  both  ends,  inserting  a  cork  in  one  end,  (or  stopping  it 
with  the  finger,)  and  immersing  the  other  in  the  water,  ihe 
water  will  not  rise  in  the  tube.  But  the  moment  the  cork  is 
taken  out,  (or  the  finger  removed,)  the  fluid  will  rise  m  the 
tube  to  a  level  with  the  surrounding  water. 

106.  The  pressure  of  a  fluid  is  in  proportion  to  the 
perpendicular  distance  from  the  surface;  that  is,  the 
deeper  the  fluid  the  greater  will  be  the  pressure.  1  his 
pressure  is  exerted  in  every  direction,  so  that  all  the 
parts  at  the  same  depth  press  each  other  with  equal 
force. 

1  A  bladder,  filled  with  air,  being  immersed  in  water,  will 
be  contracted  in  size,  on  account  of  the  pressure  of  the  water 

ranged  as  in  Fig.  52,  where  a  particle  presses  between  two  particles  be- 
neath, these  last  must  suffer  a  lateral  pressure.  In  whatever  manner  the 
particles  are  arranged,  if  they  be  globular,  as  is  suppoed,  there  must  be 
spaces  between  them.    See  Fig.  1,  page  20. 


How  long  will  the  particles  of  fluids  remain  at  rest  ?  Explani  Fig.  51. 
What  does  Fig.  52  represent?  If  the  equality  of  the  pressure  be  undis- 
turbed, what  will  follow?  If  the  fluid  be  agitated,  when  will  it  agam  come 
to  a  state  of  rest  1  How  is  the  downward  pressure  of  fluids  shown  .'  I  he 
lateral  pressure?   The  upward  pressure ? 

106  To  what  is  the  pressure  of  a  fluid  In  proportion  ?  In  what  direc- 
tion  is  this  pressure  exerted?  What  illustrations  are  given  to  prove  this? 
Why  can  a  bottle,  filled  with  water,  or  any  other  liquid,  be  let  down  to 
any  depth  without  injury  ? 


HYDROSTATIC.-^. 


79 


in  all  directions;  and  tlui  dcej)cr  it  is  immers(;d,  Uio  more  will 
it  be  contract(Hl. 

2.  An  empty  bottle,  bein;^  corked,  and  by  means  of  a  w(;i;^^lit 
let  down  to  a  certfiin  deptli  in  the  sea,  will  either  be  broken  by 
tlie  pi-essure,  or  the  cork  will  be  driven  into  it,  and  the  bottJe 
be  lilled  with  water.  This  will  take  place  even  if  the  cork  be 
secured  with  wire  and  sealed.  But  a  bottle  filled  with  water, 
or  any  other  liquid,  may  be  let  down  to  any  depth  without 
damage,  because,  in  this  case,  the  internal  pressure  is  equal  to 
the  external.* 

*  Experiments  at  sea. — We  are  indebted  to  a  friend,  who  has  just  ar- 
rived from  Europe,  says  the  Baltimore  Gazette,  for  the  following  experi- 
ments made  on  board  the  Charlemagne : 

"  26th  of  September,  183G,  the  weather  being  calm,  I  corked  an  empty 
wine-bottle,  and  tied  a  piece  of  linen  over  the  cork ;  I  then  sank  it  into 
the  sea  six  hundred  feet ;  when  drawn  immediately  up  again,  the  cork  v/as 
inside,  the  linen  remained  as  it  was  placed,  and  the  bottle  was  filled  with 
water. 

"  I  next  made  a  noose  of  strong  twine  around  the  bottom  of  the  cork, 
which  I  forced  into  the  empty  bottle,  lashed  the  twine  securely  to  the  neck 
of  the  bottle,  and  sank  the  Vjottle  six  hundred  feet.  Upon  drawing  it  up 
immediately,  the  cork  was  found  inside,  having  forced  its  way  by  the  twine, 
and  in  so  doing  had  broken  itself  in  two  pieces ;  the  bottle  was  filled  with 
water. 

I  then  made  a  stopper  of  white  pine,  long  enough  to  reach  to  the  bot- 
tom of  the  bottle  ;  after  forcing  this  stopper  into  the  bottle,  I  cut  it  off  about 
half  an  inch  above  the  top  of  the  bottle  and  drove  two  wedges,  of  the  same 
wood,  into  the  stopper.  I  sank  it  six  hundred  feet,  and  upon  drawing  it 
up  immediately  the  stopper  remained  as  I  placed  it,  and  there  Vv^as  about 
a  gill  of  water  in  the  bottle,  which  remained  unbroken.  The  water  must 
have  forced  its  way  through  the  pores  of  the  wooden  stopper,  although 
wedged  as  aforesaid  ;  and  had  the  bottle  remained  sunk  long  enough,  there 
is.  no  doubi  that  it  would  have  been  filled  with  water." 

Similar  experiments  were  made  by  the  author  of  this  work,  in  a  voyage 
to  the  West  Indies  in  the  year  1839,  first  with  an  empty  bottle  and  then 
with  a  bottle  filled  with  water  from  the  tanks  on  the  deck  ;  in  both  cases 
the  bottle  being  closely  stopped  and  the  cork  covered  with  canvass.  The 
empty  bottle  was  drawn  from  a  depth  of  six  hundred  feet  filled  with  water, 
and  the  full  bottle  with  brackish  water,  the  water  from  the  tank  having 
been  compressed,  and  water  from  the  depths  of  the  ocean  mixing  with  it. 

It  is  the  opinion  of  some  philosophers,  that  the  pressure  at  very  great 

What  experiment  is  mentioned  in  the  note  ?  What  opinion  have  some 
philosophers  expressed? 


80  NATURAL  PHILOSOPHY. 

107.  From  what  has  now  been  stated,  it  appears  that 
the  lateral  pressure  proceeds  entirely  from  the  pressure 
downwards,  or,  in  other  words,  from  the  weight  ot  the 
liquid  above  ;  and  that  consequently  the  lower  an  orihce 
is  made  in  a  vessel  containing  water  or  any  other  liquid, 
the  greater  will  be  the  force  and  velocity  with  which 
the  liquid  will  rush  out. 

Fig.  53  represents  a  vessel  of  watei', 
^itli^orilices  at  the  side  at  different  dis- 
tances from  the  surface.    The  different 
curves  in  the  figure,  described  by  the  liquid 
in  running  out  of  the  vessel,  show  the  ef- 
fects proved  bv  the  force  of  the  pres- 
sui-e  on  the  liquid  at  different  depths,  and 
the  action  of  gravity.    At  A  the  pressure  is  the  least,  because 
there  is  less  weight  of  fluid  above.    At  B  and  C  the  fluid  is 
driven  outwards  by  the  weight  of  that  portion  above,  and  «e 
force  will  be  strongest  at  C. 

108.  As  the  lateral  pressure  arises  solely  from  the 
downward  pressure,  it  is  not  aflfected  by  the  \yidth  nor 
the  length  of  the  vessel  in  which  it  is  contained,  but 
merely  bv  its  depth  ;  for  as  every  particle  acts  indepen- 
dentlv  of'the  rest,  it  is  only  the  column  of  particles  above 
the  orifice  that  can  weigh  upon  and  press  out  the  water. 

109.  The  lateral  pressure  on  one  side  of  a  cubical  ves-. 
sel  will  be  equal  only  to  half  of  the  pressure  downwards  ; 
for  every  particle  at  the  bottom  of  the  vessel  is  pressed 
upon  bv  a  column  of  the  whole  depth  of  the  fluid, 
while  the  lateral  pressure  diminishes  from  the  bottom 
upwards  to  the  surface,  where  the  particles  have  no 
pressure. 

depths  of  the  sea  is  so  great,  that  the  water  is  condensed  into  a  solid  state  ; 
and  that  at  or  near  the  centre  of  the  earth,  if  the  fluid  could  extend  so 
deeply,  this  pressure  would  convert  the  whole  into  a  solid  mass  of  fire. 

107.  What  causes  the  lateral  pressure  ?  What  follows  from  this?  Ex- 
plain Fig.  53.  r,   J  • 

108.  Does  the  length  or  the  width  of  the  vessel  in  which  a  fluid  is  con- 
tained have  any  effect  upon  the  lateral  pressure?    By  what  is  it  affected? 

109.  How  does  the  lateral  pressure  on  one  side  of  a  cubical  vessel  com- 
pare with  the  pressure  downwards  ?    How  would  you  explain  this  ? 


IIYDROSTATJCS. 


81 


110.  The  upward  pressure  of  fluids,  although  ap- 
parently ill  opposition  to  the  principles  of  gravity,  is  but 
a  necessary  consequence  of  the  operation  of  that  princi- 
ple ;  or,  in  other  words,  the  pressure  upwards  as  well  as 
the  pressure  downwards  is  caused  by  gravity. 

When  water  is  poured  into  a  vessel  with  a  spout,  (like  a 
tea-pot,  for  instance,)  the  water  rises  in  the  spout  to  a  level 
with  that  in  the  body  of  the  vessel.  The  particles  of  water  at 
the  bottom  of  the  vessel  are  pressed  upon  by  the  particles 
above  them,  and  to  this  pressure  they  will  yield,  if  there  is 
any  mode  of  making  way  for  the  particles  above  them.  As 
tliey  cannot  descend' through  the  bottom  of 
the  vessel  they  will  change  their  direction  '^^s-  5i. 

and  rise  in  the  spout.  Fig.  54  represents  a 
tea-pot,  and  the  columns  of  balls  represent 
the  particles  of  water  magnified.  From  an 
inspection  of  the  figure  it  appears  that  the 
particle  numbered  1,  at  the  bottom,  will  be 
pressed  laterally,  by  the  particle  numbered  2,  and  by  this 
pressure  forced  into  the  spout,  where  meeting  with  the  particle 
3  it  presses  it  upwards,  and  this  pressure  will  be  continued 
from  3  to  4,  from  4  to  5,  and  so  on  till  the  water  in  the  spout 
has  risen  to  a  level  with  that  in  the  body  of  the  vessel.  If 
water  be  poured  into  the  spout  the  water  will  rise  in  the  same 
manner  in  the  body  of  the  vessel ;  from  which  it  appears  that 
the  force  of  pressure  depends  entirely  on  the  height,  and  not  on 
the  length  or  breadth  of  tlve  column  of  fluid, 

111.  One  principle  in  hydrostatics,  is  so  remarkable, 
that  it  is  named  the  hydrostatic  paradox.  It  is  this. 
That  any  quantity  of  fluid ^  however  small,  may  he  made 
to  balance  and  support  any  other  quantity,  however 
large. 


110.  What  causes  the  upward  and  downward  pressure?  Illustrate  this 
by  Fig.  54. 

111.  Upon  what  does  the  force  of  pressure  depend  ?  What  is  meant  by 
the  hydrostatic  paradox?  What  is  the  use  of  the  hydrostatic  bellows? 
What  figure  represents  the  hydrostatic  bellows?  Explain  the  figure. 
What  is  the  fundamental  principle  of  Mechanics  ?  Is  this  the  principle 
of  the  hydrostatic  b^'llows  ? 

4* 


82 


NATURAL  PHILOSOPHY. 


Fig.  55 


Fig.  55  represents  the  hydrostatic  bellows.* 
A  B  is  a  long  tube,  one  inch  square.  C  D  E  F 
are  the  bellows,  consisting  of  two  boards, 
ei(/ht  inches  square,  connected  by  broad  pieces 
of  leather,  or  india-nibber  cloth,  in  the  man- 
ner of  a  pair  of  common  bellows.  One  pound 
of  water  poured  into  the  tube  will  raise  64 
pounds  on  the  bellows.  If  a  smaller  tube  be 
used  the  same  quantity  of  water  will  fill  it 
higher,  and  consequently  will  raise  a  greater 
weight ;  but  if  a  larger  tube  be  used  it  will  of 
course  not  fill  it  so  high,  and  consequently 
will  not  raise  so  great  a  weight ;  because  it 
is  the  height  not  the  quantity  lohich  causes  the 
pressure.^ 

*  This  is  the  form  of  the  Hydrostatic  bellows  in  the  original  "  Boston 
School  Set."  By  means  of  a  straight  jet  substituted  for  the  tube,  it  was 
designed  to  illustrate  a  principle  in  Hydraulics  also. 

t  The  Hydrostatic  bellows  may  be  constructed  in  a  Tariety  of  forms, 
the  simplest  of  which  consists,  as  in  the  figure,  of  two  boards  connected 
together  by  broad  pieces  of  leather,  or  india-rubber  cloth,  in  such  a  man- 
ner as  to  allow  the  upper  board  to  rise  and  fall  like  the  common  bellows. 
A  perpendicular  tube  is  so  adjusted  to  this  apparatus,  that  water  poured 
into  the  tube,  passing  between  the  boards,  will  separate  them  by  its  up- 
ward pressure,  even  although  the  upper  board  is  loaded  with  a  considerable 
weight. 

[N.  B.  A  small  quantity  of  water  must  be  poured  into  the  bellows  to  sep- 
arate the  surfaces  before  they  are  loaded  with  the  weight.] 

The  force  of  pressure  exerted  on  the  bellows  by  the  water  poured 
into  the  tube,  is  estimated  by  the  comparative  size  of  the  tube  and  the  bel- 
iov>^s.  Thus,  if  the  tube  be  one  inch  square,  and  the  top  of  the  bellows 
twelve  inches,  thus  containing  144  square  inches,  a  pound  of  water  poured 
into  the  tube  will  exert  a  pressure  of  144  pounds  on  the  bellows.  Now  it 
will  be  clearly  perceived  that  this  pressure  is  caused  by  the  height  of  the 
column  of  water  in  the  tube.  A  pound,  or  a  pint  of  water  will  fill  the 
tube  144  times  as  high  as  the  same  quantity  would  fill  the  bellows.  To 
raise  a  weight  of  144  pounds  on  the  bellows  to  the  height  of  one  inch,  it 
will  be  necessary  to  pour  into  the  tube  as  much  water  as  would  fill  the 
tube  were  it  144  Inches  long.  It  will  thus  be  perceived  that  the  funda- 
mental principle  of  the  laws  of  motion  is  here  also  in  full  force  ;  namely, 
that  what  is  gained  in  power  is  lost  either  in  time  or  in  space;  for, 
while  the  water  in  the  bellows  is  rising  to  the  height  of  one  inch,  that  in 
the  tube  passes  over  144  inches. 


HYDROSTATICS. 


83 


^ig.  5G  is  an  apparatus*  to  show  that  liquids  press  according 
'  to  the  height  and  not 

the  qiiantltij.  A  and  Fig.  56 

V)  are  two  vessels  of 
unequal  size  but  of 
tli()  same  length. 
Tliese  may  succes- 
sively be  screwed  to 
the  apparatus  and 
filled  with  water. 
Weights  may  then 
be  added  to  the  sus- 
pended scale  un- 
til the  pressure  is 
counterbalanced.  It 
will  then  be  per- 
ceived that  al- 
though A  is  ten 
times  larger  than  B, 
the  water  Avill  stand 
at  the,  same  height 

41  both,  because  they  o.re  of  the  same  length.  If  C  be  used  in- 
stead of  A  or  B,  the  apparatus  may  be  used  as  the  hydrostatic 
oellows. 

112.  If  water  be  confined  in  any  vessel,  and  a  pressure 
to  any  amount  be  exerted  on  a  square  inch  of  that 
water,  a  pressure  to  an  equal  amount  will  be  transmit- 
ted to  every  square  inch  of  the  surface  of  the  vessel  in 
which  the  water  is  confined. f 

*  This  apparatus,  belonging  to  the  "  Boston  School  Set,"  unites  sim- 
plicity with  convenience.  Instead  of  two  boards,  connected  with  leather, 
an  india-rubber  bag  is  placed  between  two  boards,  and  the  boards  are 
made  to  rise  or  fall  as  the  water  runs  into  or  out  of  the  bag.  It  is  an  ap- 
paratus easily  repaired,  and  the  bag  may  also  be  used  for  gas,  or  for  ex- 
periments in  Pneumatics. 

t  This  property  of  fluids  seems  to  invest  us  with  a  power  of  increasing 
the  intensity  of  a  pressure  exerted  by  a  comparatively  small  force,  without 
any  other  limit  than  that  of  the  strength  of  the  materials  of  which  the  en- 
gine itself  is  constructed.    It  also  enables  us  with  great  facility  to  transmit 


'  112.  What  fact  is  mentioned  in  this  number  with  regard  to  the  pres- 
sure on  water? 


S4 


NATURAL  PHILOSOPHY. 


1.  It  is  upon  this  principle  that  Bramah's  hydrostatic  press, 
represented  in  Fig.  57,  is  constructed.  The  main  features  of 
this  apparatus  are 
as  follows :  a  is  a 
narrow  and  A  a 
large  metallic  cyl- 
inder having  com- 
munication one 
with  the  other. 
Water  stands  in 
both  the  cylin- 
ders. The  piston 
S  carries  a  strong 
head  P,  which 
works  in  a  frame 
opposite  to  a  simi- 
lar plate,  R.  Be- 
tween the  two 
plates  the  sub- 
stance W  to  be 
compressed  is  placed.  In  the  narrow  tube  a  is  a  piston  p, 
worked  by  a  lever  chd,  its  short  arm  c  h  driving  the  piston, 
while  the  power  is  applied  at  d.  The  pressure  exerted  by 
the  small  piston  p  on  the  water  at  a  is  transmitted  with  equal 
force  throughout  the  entire  mass  of  the  fluid,  w^hiie  the  surface 
at  A  presses  up  the  piston  S  with  a  force  proportioned  to  its 
area.  For  instance,  if  the  cylinder  a  of  the  force-pump  has 
an  area  of  half  an  inch  while  the  great  cylinder  has  an 
area  of  200  inches,  then  the  pressure  of  the  water  in  the  lat- 
ter on  the  piston  S,  will  be  (1  half  inch  multiplied  by  400 
half  inches)  equal  to  400  times  that  on^. 

2.  Next,  suppose  the  arms  of  the  lever  to  be  to  each  other 
as  1  to  50,  and  that  at  d,  the  extremity  of  the  longer  arm,  a 

the  motion  and  force  of  one  machine  to  that  of  another,  in  cases  where  local 
circumstances  preclude  the  possibility  of  instituting  any  ordinary  mechan- 
ical connection  between  the  two  machines.  Thus,  merely  by  means  of 
water-pipes,  the  force  of  a  machine  may  be  transmitted  to  any  distance, 
and  over  inequalities  of  ground,  or  through  any  other  obstructions. 


Upon  what  principle  is  Bramah's  hydrostatic  press  constructed?  What 
figure  represents  this?  Explain  the  figure.  To  what  uses  is  this  press 
applied  ? 


HYDROSTATICS. 


85 


man  works  with  a  force  of  50  pounds,  the- piston  p  will  con- 
scMiiKMilly  descend  on  the  water  with  a  force  of  2000  pounds. 
])(;(lucliii<^r  .1  for  the  loss  of  power  caused  by  the  different  im- 
jxHliments  to  motion,  and  one  man  would  still  be  able  to  exeit 
a  force  of  three  (quarters  of  a  million  of  pounds  by  means 
of  this  machine.  This  press  is  used  in  pressing  paper,  cloth, 
hay,  gunpowder,  <kc. ;  also  in  uprooting  trees,  testing  the 
strength  of  ropes, 

113.  A  fluid  specifically  lighter  than  another  fluid  will 
float  upon  its  surface.* 

114.  A  body  specifically  lighter  than  a  fluid  will  sink 
in  the  fluid  until  it  has  displaced  a  portion  of  the  fluid 
equal  in  weight  to  itself. 

If  a  piece  of  cork  is  placed  in  a  vessel  of  water,  about  one 
third  part  of  the  cork  will  sink  below,  and  the  remainder  w^ill 
stand  above  the  surface  of  the  water ;  thereby  displacing  a 
portion  of  water  equal  in  bulk  to  about  a  third  part  of  "the 
cork,  and  this  quantity  of  water  is  equal  in  weight  to  the  whole 
of  the  cork ;  because  the  specific  gravity  of  water  is  about 
three  times  as  great  as  that  of  cork.f 

*  The  slaves  in  the  West  Indies,  it  is  said,  steal  rnm  by  inserting  the 
lonrr  neck  of  a  bottle,  full  of  water,  through  the  top  aperture  of  the  rum- 
cask.  The  water  falls  out  of  the  bottle  into  the  cask,  while  the  lighter 
mm  ascends  in  its  stead. 

t  It  is  on  the  same  principle  that  boats,  ships,  &o.,  although  composed 
of  materials  heavier  than  water,  are  made  to  float.  From  their  peculiar 
shape,  they  are  made  to  set  lightly  on  the  water.  The  extent  of  the  sur- 
face presented  to  the  water  counterbalances  the  weight  of  the  materials, 
and  the  vessel  sinks  to  such  a  depth  as  will  cause  it  to  displace  a  portion 
of  water  equal  in  weight  to  the  whole  weight  of  the  vessel.  From  a  knowl- 
edge of  the  specific  gravity  of  water,  and  the  materials  of  which  a  vessel 
is  composed,  rules  have  been  formed  by  which  to  estimate  the  tonnage  of 
vessels  ;  that  is  to  say,  the  weight  which  the  vessel  will  sustain  without 
sinking. 


113.  When  will  one  fluid  float  upon  another? 

114.  What  is  stated  with  regard  to  a  body  specifically  lighter  than  a 
fluid?  What  illustration  is  given  of  this?  How  do  the  specific  gravities 
of  water  and  cork  compare  with  each  other?  Upon  what  principle  is  it 
that  boats,  ships,  &c.,  are  made  to  float  upon  the  water?  What  rules 
have  been  formed  from  the  knowledge  of  the  specific  gravity  of  water  and 
the  materials  of  which  vessels  are  composed  ? 


86 


NATURAL  PHILOSOPHY. 


115.  The  standard  which  has  been  adopted  to  estimate 
the  specific  gravity  of  bodies,  is  rain  or  distilled  water.* 

Taking  a  certain  quantity  of  rain  or  distilled  water,  we  find 
that  a  quantity  of  gold,  equal  in  hulk,  will  weigh  nearly 
twenty  times  as  much  as  the  water ;  of  lead,  nearly  twelve 
times  as  much  ;  while  oil,  spirit,  cork,  &c.,  will  weigh  less  than 
the  water.  I 

*  As  heat  expands  and  cold  condenses  all  metals,  their  specific  gravity 
cannot  be  the  same  in  summer  that  it  is  in  winter.  For  this  reason  they 
will  not  serve  as  a  standard  to  estimate  the  specific  gravity  of  other  bodies. 
The  reason  that  distilled  water  is  used  is,  that  spring,  well,  or  river  water 
is  seldom  perfectly  pure  ;  and  the  various  substances  mixed  with  it  affect 
its  weight.  The  cause  of  the  ascent  of  steam,  or  vapor,  may  be  found  in 
its  specific  gravity.  It  may  here  be  stated  that  rain,  snow,  and  hail  are 
formed  by  the  condensation  of  the  particles  of  vapor  in  the  upper  regions 
of  the  atmosphere.  Fine  watery  particles  coming  within  the  sphere  of 
each  other's  attractions,  unite  in  the  form  of  a  drop,  which  being  heavier 
than  the  air,  falls  to  the  earth.  Snow  and  hail  differ  from  rain  only  in  the 
different  degrees  of  temperature  at  which  the  particles  unite.  When  rain, 
snow,  or  hail  fall,  part  of  it  reascends  in  the  form  of  vapor,  and  forms 
clouds ;  part  is  absorbed  by  the  roots  of  vegetables,  and  part  descends  into 
the  earth  and  forms  springs.  The  springs  form  brooks,  rivulets,  rivers,  &c., 
and  descend  to  the  ocean,  where  being  again  heated  by  the  sun,  the  water 
rising  in  the  form  of  vapor,  again  forms  clouds,  and  again  descends  in  rain, 
snow,  hail,  The  specific  gravity  of  the  watery  particles  which  con- 

stitute vapor,  is  less  than  that  of  the  air  near  the  surface  of  the  earth ; 
they  will,  therefore,  ascend  until  they  reach  a  portion  of  the  atmosphere  of 
the  same  specific  gravity  with  themselves.  Bat  the  constant  accession  of 
fresh  vapor  from  the  earth,  and  the  loss  of  heat,  causes  several  particles  to 
come  within  the  sphere  of  each  other's  attraction,  as  has  been  stated  above, 
and  they  unite  in  the  form  of  a  drop,  the  specific  gravity  of  which  being 
greater  than  that  of  the  atmosphere,  it  will  fall  in  the  form  of  rain.  Water, 
as  it  descends  in  rain,  snow,  or  hail,  is  perfectly  pure,  but  when  it  has 
fallen  to  the  earth,  it  mixes  with  the  various  substances  through  which  it 
passes,  which  gives  it  a  species  of  flavor,  without  affecting  its  transparency. 

t  The  following  table  shows  the  specific  gravity  of  the  substances  there- 
in mentioned.  It  is  to  be  understood  that  all  substances  whose  specific 
gravity  is  greater  than  water,  will  sink  when  immersed  in  it,  and  that  all 


^  •  115.  What  standard  has  been  adopted  to  estimate  the  specific  gravity  of 
substances  in  general?  Why  could  not  metals  have  been  adopted  ?  Why 
js  distilled  water  used  1  What  bodies  will  sink  when  immersed  iii  water  ? 
What  will  float? 


IIVDUOSTATICS. 


87 


Fig.  58. 


IK).  Tlio  spec/ilic  gnivily  of  bodies  that  will  sink  in 
water  is  ascertained  by  weii;Innu^  tbeni  lirst.  in  water, 
and  then  out  of  the  water,  Jind  dividing  the  weiijht  out 
ot'the  water  by  the  loss  oi' weight  in  water. 

1.  Fig'.  58  represents  the  scales  for  as- 
certaining tlie  specilic  gravity  of  bodies. 
One  scale  is  shorter  than  the  other,. and 
a  hook  is  attached  to  the  bottom  of  the 
scale  to  which  substances,  whose  speci- 
fic gravity  is  sought,  may  be  attached 
and  sunk  in  water. 

2.  Suppose  a  cubic  inch  of  gold 
w^eighs  19  ounces  when  weighed  out 

whose  specific  gravity  is  less  than  that  of  water,  will  float  in  it.  Let  us 
then  take  a  quantity  of  water  which  will  weigh  exactly  one  pound  ;  a 
quantity  of  the  substances  specified  in  the  table,  of  the  same  bulk,  will 
weio^h  as  follows : 


Piatinum, 
Fiiie  Gold,  - 

23.  lbs. 

Chalk,  -  - 

-    1.793  lbs. 

19.640  " 

Coal,     -  - 
Mahogany, 

-    1.250  " 

Mercury, 

14.019 

-    1.063  " 

Lead,  -    -  - 

11.525  " 

Milk,     - '  - 

■    1.034  " 

Silver,     -  - 
Copper,   -  - 

11.091  " 

Box-wood, 

1.030  " 

9.000  " 

Ram  water, 

-    1.000  " 

Iron,    -    -  - 
Glass,  -    -  - 

7.645  " 

Oil,  -   -  - 
Ice,  -   -  - 

-     .920  " 

3.000  " 

-     .908  " 

iMarble,    -  - 

2.705  " 

Brandy, 

-     .820  " 

Living  men,  - 

Ash,  -   -   -  - 

Beech,    -   -  - 

Elm,  -    -   -  - 

Fir,    -   -   -  . 

Cork,  -   -   -  . 

Common  air,  ■ 
Hydrogen  gas, 


.891  lbs. 
.800  " 
.700  " 
.600  " 
.500  " 
.240  " 
.0011  " 
.000105 


A  cubic  foot  of  water  weighs  one  thousand  avoirdupois  ounces.  By 
multiplying  the  number  opposite  to  any  substance  in  the  above  table  by  one 
thousand,  we  obtain  the  weight  of  a  cubic  foot  of  that  substance,  in  ounces 
Thus,  a  cubic  foot  of  platinum  is  23000  ounces  in  weight. 

In  the  above  table  it  appears  that  the  specific  gravity  of  living  men  is 
about  one  ninth  less  than  that  of  common  water.  So  long,  therefore,  as  the 
lungs  can  be  kept  free  from  water,  a  person,  although  unacquainted  with 
the  art  of  swimming,  will  not  completely  sink,  provided  the  hands  and  arms 
be  kept  under  water. 

The  specific  gravity  of  sea  water  is  greater  than  that  of  the  water  of 
lakes  and  rivers,  on  account  of  the  salt  contained  in  it.  On  this  account 
the  water  of  lakes  and  rivers  has  less  buoyancy,  and  it  is  more  diffiult  to 
swim  in  it. 


What  is  the  weight  of  a  cubic  foot  of  water?  What  is  the  use  of  the 
above  table  ?  How  does  the  specific  gravity  of  living  men  compare  with 
that  of  water?  Which  is  the  greater,  the  specific  gravity  of  sea  water,  or 
of  lakes  and  rivers?  Why'^ 

116.  How  is  the  specific  gravity  of  bodies  that  will  sink  in  water  ascer- 
tained?   What  illustration  is  given?    Explain  Fig  58 


88 


NATURAL  PHTLOSOPHY, 


of  the  water,  and  but  18  ounces'^  when  weighed  in  water — the 
loss  in  water  is  one  ounce.  The  weight  out  of  water,  19 
ounces,  being  divided  by  one  (the  loss  in  water)  gives  19. 
The  specific  gravity  of  gold,  then,  would  be  19,  or,  in  other 
words,  gold  is  nineteen  times  heavier  than  water. 

117.  The  specific  gravity  of  a  body  that  will  not  sink 
in  water,  is  ascertained  by  dividing  its  weight  by  the 
sum  of  its  weight,  added  to  the  loss  of  weight  which 
it  occasions  in  a  heavy  body  previously  balanced  in 
water,  f 

*  The  gold  will  weigh  less  in  the  water  than  out  of  it,  on  account  of  the 
upward  pressure  of  the  particles  of  water,  which  in  some  measure  supports 
it,  and  by  so  doing  diminishes  its  weight.  Now,  as  the  upward  pressure 
of  these  particles  is  exactly  sufficient  to  balance  the  downward  pressure  of 
a  quantity  of  water  of  exactly  the  same  dimensions  with  the  gold,  it  fol- 
lows that  the  gold  will  lose  exactly  as  much  of  its  weight  in  water,  as  a 
quantity  of  water  of  the  same  dimensions  with  the  gold  will  weigh.  And 
this  rule  applies  to  all  bodies  heavier  than  water,  that  are  immersed 
in  it.  They  will  lose  as  much  of  their  weight  in  water  as  a  quan- 
tity of  water  of  their  own  dimensions  weighs.  All  bodies,  therefore,  of 
the  same  size,  lose  the  same  quantity  of  their  weight  in  water.  Hence, 
tlie  specific  gravity  of  a  body  is  the  weight  of  it  compared  with  that 
of  water.  As  a  body  loses  a  quantity  of  its  weight  when  immersed  in 
water,  it  follows  that  w^hen  the  body  is  lifted  from  the  water,  that  portion 
of  its  weight  which  it  had  lost  will  be  restored.  This  is  the  reason  that 
a  bucket  of  water,  drawn  from  a  well,  is  heavier  when  it  rises  above  the 
surface  of  the  water  in  the  well,  than  it  is  while  it  remains  below  the  sur- 
face.   For  the  same  reason  our  limbs  feel  heavy  in  leaving  a  bath. 

t  The  method  of  ascertaining  the  specific  gravities  of  bodies  was  dis- 
covered accidentally  by  Archimedes.  He  had  been  employed  by  the  king 
of  Syracuse  to  investigate  the  metals  of  a  golden  crov/n  which  he  suspect- 


^Vhy  will  gold  weigh  less  in  the  water  than  out  of  it  ?  How  does  this 
upward  pressure  of  the  particles  compare  with  the  downward  pressure  of 
a  quantity  of  water  of  the  same  dimensions?  What  follows  from  this? 
What  rule  is  given  with  regard  to  all  bodies  heavier  than  water  that  are 
immersed  in  it  ?  What  is  the  specific  gravity  of  a  body  ?  What  is  the 
reason  that  a  bucket  of  water,  drawn  from  a  well,  is  heavier  when  it 
rises  above  the  surface  of  the  water,  than  while  it  is  below  it  ? 

117.  How  can  the  specific  gravity  of  a  body  that  will  not  sink  in  water 
be  ascertained?  What  illustration  is  given?  By  whom  Wds  the  method 
of  ascertaining  the  specific  gravities  of  bodies  discovered  ?  In  what  map- 
nev  did  he  ascertain  it? 


II  Y  l>K()S'l\\  ricB. 


89 


If  Ji  body  lii^lilcr  (lian  w.-ilcr  weighs  six  ounces,  and  on  ho- 
lug  attaclK.'d  (o  a  heavy  body,  balanced  in  water,  is  found  to 
occasion  it  to  lose  twelve  ounces  of  its  weight,  its  sj)(H^ili(' 
o'lavity  is  determined  by  dividing  its  weight  (six  ounces)  by 
the  sum  of  its  Aveight,  added  to  the  loss  of  weight  it  occasions 
in  the  heavy  body,  namely,  six  added  to  twelve,  which,  in 
other  words,  is  6  divided  by  18,  or       whicli  is  1. 

118.  An  hydrometer  is  an  instrument  to  ascertain  tlie 
specific  gravity  of  liquids. 

1.  The  hydrometer  is  constructed  on  the  principle,  that 
the  greater  the  wx^ight  of  a  liquid,  the  greater  will  be  its 
buoyancy. 

2.  The  hydrometer  is  made  in  a  variety  of  forms,  but  it 
generally  consists  of  a  hollow  ball  of  silver,  glass,  or  other 
material,  with  a  graduated  scale  rising  from  the  upper  part. 
A  weight  is  attached  below^  the  ball.  When  the  instrument, 
thus  constructed,  is  immersed  in  a  fluid,  the  specific  gravity 
of  the  fluid  is  estimated  by  the  portion  of  the  scale  that  re- 
mains above  the  surface  of  the  fluid.  The  greater  the  specific 
gravity  of  the  fluid,  the  less  will  the  scale  sink. 

ed  had  been  adulterated  by  the  w-orkmen.  The  philosophei  labored  at  the 
problem  in  vain,  till  going  one  day  into  the  bath,  he  perceived  that  the 
water  rose  in  the  bath  in  proportion  to  the  bulk  of  his  body.  He  instantly 
perceived  that  any  other  substance  of  equal  size  would  raise  the  water  just 
as  much,  though  one  of  equal  weight  and  less  bulk  could  not  produce  the 
same  effect.  He  then  obtained  two  masses,  one  of  gold  and  one  of  silver, 
each  equal  in  weight  to  the  crown,  and  having  filled  a  vessel  very  accu- 
rately with  water,  he  first  plunged  the  silver  mass  into  it,  and  observed  the 
quantity  of  water  that  flowed  over ;  he  then  did  the  same  with  the  gold, 
and  found  that  a  less  quantity  had  passed  over  than  before.  Hence  he 
inferred  that,  though  of  equal  weight,  the  bulk  of  the  silver  was  greater 
than  that  of  the  gold,  and  that  the  quantity  of  water  displaced  was,  in 
each  experiment,  equal  to  the  bulk  of  the  metal.  He  next  made  trial 
with  the  crown,  and  found  that  it  displaced  more  water  than  the  gold,  and 
less  than  the  silver,  which  led  him  to  conclude,  that  it  was  neither  pure 
gold  nor  pure  silver. 


118.  What  is  an  hydrometer?  Upon  what  principle  is  it  constructed? 
Explain  its  construction.    In  what  proportion  does  the  scale  sink  ? 


90 


NATURAL  PHILOSOPHY 


CHAPTER  YI. 

HYDRAULICS. 

119.  Hydraulics  treats  of  the  motion  of  fluids,  particu- 
larly of  water ;  and  the  construction  of  all  kinds  of  in- 
struments and  machines  for  moving  them. 

Water,  in  its  motion,  is  retarded  by  the  friction  of  the 
bottom  and  sides  of  the  channel  through  which  it  passes. 
For  this  reason  the  velocity  of  the  surface  of  a  running  stream 
is  always  greater  than  that  of  any  other  part.^ 

120.  A  fluid  running  from  an  orifice  in  a  vessel  is  dis- 
charged wdth  the  greater  rapidity  when  the  vessel  from 
which  it  flow^s  is  kept  constantly  full. 

This  is  a  necessary  consequence  of  the  law,  that  pressure  is 
proportioned  to  the  height  of  the  column  above. 

121.  When  a  fluid  spouts  from  several  orifices  in  the 
side  of  a  vessel,  it  is  throw^n  to  the  greatest  distance 
from  the  orifice  nearest  to  the  centre. t 

122.  A  vessel  filled  with  any  liquid  will  discharge  a 
greater  quantity  of  the  liquid  through  an  orifice  to  which 

*  In  consequence  of  the  friction  of  the  banks  and  beds  of  rivers,  and  the 
numerous  obstacles  they  meet  in  their  circuitous  course,  their  progress  is 
slow.  If  it  were  not  for  these  impediments,  the  velocity  which  the  waters 
would  acquire  would  produce  very  disastrous  consequences.  An  inclina- 
tion of  three  inches  in  a  mile,  in  the  bed  of  a  river,  will  give  the  current  a 
velocity  of  about  three  miles  an  hour. 

t  This  is  true  only  on  the  condition  that  the  vessel  be  not  elevated.  If 
the  vessel  be  elevated,  the  lowest  orifice  will  discharge  the  fluid  to  the 
greatest  distance,  but  when  the  vessel  is  placed  low,  the  fluid  will  reach 
the  plane  before  its  projectile  force  is  expended. 

119.  Of  what  does  Hydrauhcs  treat  ?  What  retards  the  motion  of  water  ? 
Why  does  the  surface  of  a  canal  or  river  have  a  greater  velocity  than  any 
other  part  ? 

120.  Does  the  fulness  of  a  vessel  from  the  orifice  of  which  a  fluid  is 
running,  have  any  effect  upon  its  velocity  ? 

121.  When  a  fluid  spouts  from  several  orifices  in  the  side  of  a  vessel, 
from  'which  is  it  thrown  to  the  greatest  distance  ? 


HYDRAULICS. 


91 


a  short  pipe  of  peculiar  shape  is  fitted,  than  through  an 
orifice  of  the  same  size  without  a  pipe.*  But  if  the  pipe 
project  into  the  vessel,  the  quantity  discharged  will  be 
diminished  instead  of  increased  by  the  pipe. 

The  quantity  of  a  fluid  discharged  through  a  pipe  or  an 
orifice  is  increased  by  heating  the  liquid ;  because  heat  di- 
minishes the  cohesion  of  the  particles,  which  exists,  to  a  certain 
degree,  in  all  liquids. 

123.  The  velocity  of  a  current  of  v^ater  may  be  as- 
certained by  immersing  in  it  a  bent 
tube,  shaped  like  a  tunnel  at  the  end 
which  is  immersed. 

Fig.  59  is  a  tube'  shaped  like  a  tun- 
nel, with  the  larger  end  immersed  in  an 
opposite  direction  to  the  current.  The 
rapidity  of  the  current  is  estimated  by 
the  height  to  which  the  water  is  forced 
into  the  tube,  above  the  surface  of  the 
current.  By  such  an  instrument  the 
comparative  velocity  of  different  streams, 
or  the  same  stream  at  different  times, 
may  be  estimated. f 


Fig.  59 


*  This  is  caused  by  the  cross-currents  made  by  the  rushing  of  the  water 
from  different  directions  towards  the  sharp-edged  orifice.  The  pipe  smooths 
the  passage  of  the  Hquid. 

t  To  measure  the  velocity  of  a  stream  at  its  surface,  hollow  floating 
bodif^s  are  used ;  as,  for  example,  a  glass  bottle  filled  with  a  sufficient 
quaptity  of  water  to  make  it  sink  just  below  the  level  of  the  current, 
and  having  a  small  flag  projecting  from  the  cork.  A  wheel  may  also  be 
cau?ed  to  revolve  by  the  current  striking  against  boards  projecting  from 
the  circumference  of  the  wheel,  and  the  rapidity  of  the  current  may  be 
estimated  by  the  number  of  the  revolutions  in  a  given  time. 


122.  What  effect  will  a  pipe,  fitted  to  an  orifice,  have  with  regard  to  the 
quantity  discharged?  What  will  be  the  effect  if  the  pipe  project  into  the 
vessel  ?  How  can  the  quantity  discharged  through  a  pipe  or  orifice,  be 
increased?    Why  will  heat  increase  it? 

123.  How  can  the  velocity  of  a  current  of  water  be  ascertained?  Wh^. 
does  Fig.  59  represent  ?    How  is  the  rapidity  of  the  current  estimated 
What  is  the  use  of  the  instrument  ? 


92 


NATURAL  PHILOSOPHY. 


124.  Waves  are  caused  by  the  friction  between  air 
and  water.* 

125.  The  instruments  used  for  raising  or  drawing 
water  or  other  Hquids,  are  the  common  pump,t  the 
chain  pump,  the  forcing  pump,  the  siphon,  and  the  screw 
of  Archimedes. 

126.  The  chain-pump  is  a  ma- 
chine by  which  the  water  is  hfted 
through  a  box  or  channel,  by  boards 
fitted  to  the  channel  and  attached 
to  a  chain.  It  has  been  used  prin- 
cipally on  board  of  ships. 

Fig.  60  represents  a  chain-pump.  It 
consists  of  a  square  box  through  which 
a  number  of  square  boards  or  buckets, 
connected  by  a  chain,  is  made  to  pass. 
The  chain  passes  over  the  wheel  C  and 
under  the  wheel  D,  which  is  under 
water.  The  buckets  are  made  to  fit 
the  box,  so  as  to  move  with  little  fric- 
tion. The  upper  wheel,  C,  is  turned 
by  a  crank,  (not  represented  in  the  Fig.) 
which  causes  the  chain  with  the  buckets 
attached  to  pass  through  the  box.  Each 
bucket,  as  it  en:ers  the  box,  lifts  up  the 
water  above  it,  and  discharges  it  at  the 
top. 

127.  The  screw  of  Archimedes  is  a  machine  said  to 
have  been  invented  by  the  philosopher  Archimedes,  for 

*  When  oil  is  poured  on  the  windward  side  of  a  pond,  the  whole  surface 
will  become  smooth.  The  oil  protects  the  water  from  the  friction  of  the 
wind  or  air.  It  is  said  that  boats  have  been  preserved  in  a  raging  surf,  in 
consequence  of  the  sailors  having  emptied  a  barrel  of  oil  on  the  water. 

t  The  common  pump,  and  the  forcing  pump,  will  be  explained  in  con- 
nection with  Pneumatics. 

124.  What  causes  waves?  What  is  sometimes  done  to  remove  this 
friction  ? 

125.  What  instruments  are  used  for  ra:sing  liquids  ? 

126.  W^here  is  the  chain-pump  used  ?  What  figure  represents  it  ?  Ex- 
plain the  figure. 


HYDRAULICS. 


93 


raising  water  and  draining  the  lands  of  Egypt,  about 
200  years  before  the  Christian  era. 
Fisr.  6 1  represents  the  „. 

r     A     1  •    -J  ^^S'  61. 

screw  01  Archimedes. 
A  single  tube,  or  two 
tubes,  are  wound  in  the 
form  of  a  screw  around 
a  shaft  or  cylinder,  sup- 
ported by  the  prop  and 
the  pivot  A,  and  turned 
by  the  handle  n.  As 
the  end  of  the  tube  dips 
into  the  water,  it  is  filled 
with  the  fluid,  which  is 
forced  up  the  tube  by 
every  successive  revolution,  until  it  is  discharged  at  the  upper 
end. 

128.  Springs  and  rivulets  are  formed  by  the  water, 
from  rain,  snow,  &c.,  which  penetrates  the  earth,  and 
descends  until  it  meets  a  substance  which  it  cannot  pen- 
etrate. A  reservoir  is  then  formed  by  the  union  of  small 
streams  under  ground,  and  the  water  continues  to  accu- 
mulate until  it  finds  an  outlet. 

Fig.  62. 


Fig.  62  represents  a  vertical  section  of  the  crust  of  the 
earth,    a,  c,  and  e  are  strata,  either  porous,  or  full  of  cracks, 


127.  What  is  said  of  the  screw  of  Archimedes?  Explain  the  use  of  the 
screw  by  Fig.  61. 

128.  How  are  springs  and  rivulets  formed  ?    Explain  Fi^.  62. 


94 


NATURAL  PHILOSOPHY. 


vTaich  permit  the  water  to  flow  through,  while  h,  d,  and/,  are 
impervious  to  the  water.  Xow  according  to  the  laws  of  hy- 
drostatics, the  water  at  h  will  descend  and  form  a  natural 
sprino-  at  g  ;  at  i  it  will  run  with  considerable  force,  formmg  a 
natuml  jet ;  and  at  p,  and  g,  artesian  wells  may  be  dug,  in 
which  the  water  will  rise  to  the  respective  heights  g  h,  pK  and 
I  m,  the  water  not  being  allowed  to  come  in  contact  with  the 
porous  soil  through  which  the  bore  is  made,  but  being  brought 
in  pipes  to  the  surface ;  at  n  the  water  will  ascend  to  about  o, 
and  there  will  be  no  fountain.  This  explains  also  the  manner 
in  which  water  is  obtained  by  digging  wells. 

129.  A  spring  will  rise  nearly  as  high,  but  cannot  rise 
hio-her  than  the  reservoir  from  whence  it  issues.  Water 
may  be  conveved  over  hills  and  valleys  in  bent  pipes 
and  tubes,  or  "through  natural  passages,  to  any  height 
which  is  not  greater  than  the  level  of  the  reservoir  Irom 
whence  it  flows.* 

130.  Fountains  are  formed  by  water 
carried  through  natural  or  artificial 
ducts  from  a  reservoir.  The  water  will 
spout  through  the  ducts  to  nearlyt  the 
height  of  the  surface  of  the  reservoir. 

A  simple  method  of  making  an  artificial 
fountain  may  be  understood  by  Fig.  63. 
A  glass  siphon  a  5  c  is .  immersed  in  a  vessel 
of  water,  and  the  air  being  exhausted  from 
the  siphon,  a  jet  will  be  produced  at  a,  pro- 
portioned to  the  fineness  of  the  bore  and  the 
length  of  the  tube. 


Fig.  63. 


*  The  ancient  Romans,  ignorant  of  this  property  of  fluids,  constructed 
vast  aqueducts  across  valleys,  at  great  expense,  to  convey  water  over  them. 
The  moderns  effect  the  same  object  by  means  of  wooden,  metaUic,  or 
stone  pipes. 

t  The  resistance  of  the  air  prevents  the  fluids  from  rising  to  quite  the 
same  height  with  the  reservoir. 


129.  How  high  will  a  spring  rise  ? 

1,30.  How  are  fountains  formed?  How  high  will  the  water  spout 
through  the  ducts?  What  prevents  the  fluid  from  rismg  to  the  same 
height  with  the  reservoir  ? 


HYDRAULICS. 


95 


131.  The  siphon  is  a  tube  bent  in  the  form  of  the 
letter  U,  one  side  being  a  Httle  longer  than  the  other. 

1.  Fig.  64  represents  a  siphon.    A  siphon  is 

used  by  filhng  it  witli  water  or  some  other  fluid,  ^^s-  64. 
then  stopping  both  ends,  and  in  this  state  im- 
mersing the  shorter  leg  or  side  into  a  vessel  con- 
taining a  liquid.  The  ends  being  then  unstopped, 
the  licpiid  will  run  through  the  siphon  until  the 
vessel  is  emptied.  In  performing  this  experiment, 
the  end  of  the  siphon  which  is  out  of  the  water  must 
always  he  below  the  surface  of  the  water. 

2.  This  instrument  may  also  be  used  to  show 
the  equilibrium  of  fluids.  For,  if  the  tube  be  in- 
verted and  two  liquids  poured  into  it,  they  will 
rise  in  each  side  or  leg  of  the  siphon  to  dif- 
ferent heights — the  higher  fluid  standing  at  the  highest  level. 
The  specific  gravity  of  mercury  being  thirteen  times  greater  than 
that  of  Avater,  will  balance  thirteen  times  its  bulk  of  water. 
Consequently  water  will  rise  thirteen  times  as  high  on  one  side 
of  the  siphon  as  the  mercury  on  the  other.  But  if  one  liquid 
only  is  poured  into  the  siphon  it  will  rise  to  the  same  height  in 
both  sides  or  legs  of  the  siphon. 

3.  Tantalus'  cup  consists  of  a  goblet  containing  a  small 
figure  of  a  man.    A  siphon  is  concealed  within 

the  figure,  which  empties  the  water  from  the  ^5. 
goblet  as  fast  as  it  is  poured  in,  so  that  the  glass 
can  never  be  filled. 

4  Fig.  65  represents  the  cup  with  the  siphon. 
The  figure  of  the  man  is  omitted,  in  order  that 
the  position  of  the  siphon  may  be  seen. 

132.  Water,  by  means  of  its  weight  or  its  force  when 
in  motion,  becomes  a  mechanical  agent  of  great  power. 
It  is  used  to  propel  or  turn  wheels  of  different  construc- 
tion, which,  being  connected  with  machinery  of  various 
kinds,  form  mills,  &c. 


131.  What  is  the  siphon  ?  In  what  manner  is  the  siphon  used?  How 
can  the  siphon  be  used  to  show  the  equilibrium  of  fluids  ?  How  high  will 
the  liquid  rise  in  each  side  of  the  siphon  ?  What  is  Tantalus'  cup  ?  What 
does  Fig.  65  represent  ? 

132.  How,  and  for  what  purposes  is  water  used  as  a  mechanical  agent  ? 


96 


NATURAL  PHILOSOPHY. 


There  are  three  kinds  of  water-wheels,  called  un- 
dershot, overshot,  and  breast  wheels. 

133.  The  Overshot  wheel  is  a  wheel  set  in  motion  by 
the  weight  of  water  flowing  upon  it.  It  receives  its 
motion  at  the  top. 

Fig.  66  represents  the  overshot 
wheel.  It  consists  of  a  wheel  turn- 
ing on  an  axis,  (not  represented  in 
the  Fig.,)  with  compartments  called 
buckets,  ahcd,  &c.,  at  the  circum- 
ference, which  are  successively  filled 
with  water  from  the  stream  S.  The 
weight  of  the  water  in  the  buckets 
causes  the  wheel  to  turn,  and  the 
buckets  being  gradually  inverted  are 
emptied  as  they  descend.  It  will  be  seen,  from  an  inspection 
of  the  figure,  that  the  buckets  in  the  descending  side  of  the 
wheel  are  always  filled,  or  partly  filled,  while  those  in  the  op- 
posite or  ascending  part  are  always  empty  until  they  are  agam 
presented  to  the  stream.  This  kind  of  wheel  is  the  most 
powerful  of  all  the  water-wheels. 

134.  The  Undershot  wheel  is  a  wheel  which  is  moved 
by  the  motion  of  the  water.  It  receives  its  impulse  at 
the  bottom. 

Fig.  67  represents  the 
undershot  wheel.  Instead 
of  buckets  at  the  circum- 
ference, it  is  furnished  with 
plane  surfaces,  called  float- 
boards,  ahcd,  &c.,  which 
receive  the  impulse  of  the 
water,  and  cause  the  wheel 
to  revolve. 


How  many  kinds  of  water-wheels  are  there?    What  are  they? 

133.  What  is  the  overshot  wheel?  Where  does  it  receive  its  motion? 
Explain  Fig.  66.  What  causes  the  wheel  to  turn?  How  does  this  wheel 
compare  in  power  with  the  other  water-wheels? 

134.  What  is  the  undershot  wheel?  Where  does  it  receive  its  motion? 
What  does  Fig.  67  represent?  How  does  this  wheel  differ  from  the 
overshot  ? 


rNEi;MATICS. 


97 


135.  The  Breast-wheel  is  a  wheel  which  receives  the 
water  at  about  half 
its  own  height,  or  ^'s- 
at  the  level  of  its 
axis.  It  is  moved 
both  by  the  weight 
and  the  motion  of 
the  w^ater.* 

Fig.  68  represents  a 
breast-wheel.  It  is 
furnished  either  with 
buckets,  or  with  float- 
boards,  fitting  the  water-course. 


CHAPTER  YII. 

PNEUMATICS. 

136.  Pneumatics  treats  of  the  nature,  mechanical 
properties,  and  effects  of  air  and  similar  fluids,  distin- 
guished by  the  name  of  aeriform  fluids,  f 

137.  The  air  we  breathe  is  an  elastic  fluid  which  sur- 
rounds the  earth,  extending  to  an  indefinite  distance 
above  its  surface,  and  constantly  decreasing  upwards  in 
density.  J 

*  In  all  the  wheels  which  have  been  described,  the  motion  given  to  the 
wheel  is  communicated  to  other  machinery  or  gearing,  as  it  is  called,  by 
)ther  wheels  or  pinions  attached  to  the  axis,  such  as  have  been  described 
under  the  head  of  Mechanics. 

t  An  aeriform  fluid  is  a  fluid  in  the  form  of  air,  and,  like  air,  generally 
nvisible. 

t  The  terms  "  rarefaction"  and  "  rarefied"  are  applied  to  air  when  it  is 
axpanded ;  and  "  condensation"  or  "  condensed  "  when  it  is  compressed. 

135.  What  is  the  breast-wheel  ?  How  is  it  set  in  motion?  What  figure 
represents  the  breast-wheel  ?  To  what  is  the  motion  given  to  the  wheels 
which  have  been  described,  communicated? 

136.  Of  what  does  Pneumatics  treat? 

137.  What  is  the  air  which  we  breathe  /  How  far  does  it  extend  above 
the  surface  of  the  earth  ? 

5 


gg  NATURAL  PHILOSOPHY. 

It  possesses  many  of  tlie  properties  belonging  to  liquids  in 
general,  besides  several  others,  the  result,  or,  perhaps,  the 
cause  of  its  elasticity.  Its  specific  gravity  is  eight  hundred 
times  less  than  that  of  water. 

138.  Air,  steam,  vapor,  gas,  are  all  elastic  fluids  pos- 
sessing the  same  mechanical  properties. f  Whatever, 
therefore,  is  stated  in  relation  to  air,  belongs  m  common 
to  all  of  these  fluids. 

139.  Aeriform  fluids  have  weight,  but  no  cohesive 
attraction. 

140.  Air  has  two  principal  properties,  namely.  Grav- 
ity and  Elasticity-J 

It  has  already  been  stated,  that  the  air  near  the  surface  of  the  earth  bears 
the  weight  of  that  which  is  above  it.  Being  compressed,  therefore,  by  the 
weight  of  that  above  it,  it  must  exist  in  a  condensed  form  near  the  surface 
of  the  earth,  while  in  the  upper  regions  of  the  atmosphere,  where  there  is 
no  pressure,  it  is  highly  rarefied.  This  condensation,  or  pressure,  is  very 
similar  to  that  of  water  at  great  depths  in  the  sea. 

*  The  air  is  necessary  to  animal  and  vegetable  life,  and  to  combustion. 
It  is  a  very  heterogeneous  mixture,  being  filled  with  vapors  of  all  kinds 
It  consists,  however,  of  two  principal  ingredients,  called  oxygen  and  nitro- 
gen, or  azote ;  of  the  former  of  which  there  are  28  parts,  and  of  the  latter, 
72  in  a  hundred.  The  air  is  not  visible,  because  it  is  perfectly  transparent. 
It  may  be  felt  when  it  moves  in  the  form  of  wind,  or  by  swinging  the  hand 
rapidly  backwards  and  forwards.  .  . 

t  The  chemical  properties  of  liquids,  fluids,  &c.,  are  not  treated  m  the 
sciences  of  Pneumatics,  Hydraulics,  or  Hydrostatics,  but  belong  peculiarly 
to  the  science  of  Chemistry.  They  are  not,  therefore,  described  m  this 
work.  But  fluids  possess  all  the  properties  of  liquids,  and  the  laws  of 
Hydrostatics  and  Hydraulics  apply  to  them  as  well  as  to  liquids. 

X  Besides  these  two  j>rincipal  properties,  the  operations  of  which  pro- 
duce most  of  the  phenomena  of  Pneumatics,  it  will  be  recollected  that 
as  air,  although  an  invisible  is  yet  a  material  substance,  possessing  all  the 

Does  it  possess  properties  common  to  liquids  in  general  i  How  does  its 
specific  gravity  compare  with  that  of  water  ?  Of  what  two  principal  ingre- 
dients does  the  air  consist  ?    What  is  the  proportion  of  these  parts  to  each 

""^^38.  What  other  fluids  are  named  belonging  to  the  class  of  elastic  fluids? 

139.  Have  the  air  and  other  similar  fluids  weight?  With  what  power 
alone  has  heat  to  contend  in  aeriform  fluids? 

140  What  two  principal  properties  has  the  air  ? 


PNEUMATICS. 


99 


141.  A  column  of  air,  having  a  base  an  inch  square, 
and  reaching  to  the  top  of  the  atmosphere,  weighs 
about  fifteen  pounds.  This  pressure,  hke  the  pressure 
of  Hquids,  is  exerted  equally  in  all  directions.* 

142.  The  elasticity  of  air  and  other  aeriform  fluids  is 
that  property  by  which  they  are  increased  or  diminish- 
ed in  extension,  according  as  they  are  compressed. f 

This  property  exists  in  a  much  greater  degree  in  air  and 
other  similar  fluids  than  in  any  other  substance.  In  fact  it 
has  no  known  limit ;  for  when  the  pressure  is  removed  from 
any  portion  of  air,  it  immediately  expands  to  such  a  degree 
that  the  smallest  quantity  will  diffuse  itself  over  an  indefinitely 
lai-ge  space.  ^  And,  on  the  contrary,  when  the  pressure  is  in- 
creased, it  will  be  compressed  into  indefinitely  small  dimen- 
sions.J 

common  properties  of  matter,  it  possesses  also  the  common  property  of 
imp enetr ability.    This  will  be  illustrated  by  experiments. 

*  It  has  been  computed  that  the  weight  of  the  whole  atmosphere  is 
equal  to  tnat  of  a  globe  of  lead  sixty  miles  in  diameter,  or  to  five  thousand 
billions  of  tons. 

t  The  pressure  of  the  atmosphere  caused  by  its  weight  is  exerted  on  all 
substances,  internally  and  externally,  and  it  is  a  necessary  consequence  of 
its  fluidity.  The  body  of  a  man  of  common  stature  has  a  surface  of  about 
2000  square  inches ;  whence  the  pressure  at  15  pounds  per  square  inch 
will  be  30,000  pounds.  The  reason  why  this  immense  weight  is  not  felt, 
is,  that  the  air  within  the  body  and  its  pores  counterbalances  the  weight 
of  the  external  air.  When  the  external  pressure  is  artificially  removed 
from  any  part,  it  is  immediately  felt  by  the  reaction  of  the  internal  air. 

X  Heat  insinuates  itself  between  the  particles  of  bodies,  and  forces  them 
asunder,  in  opposition  to  the  attraction  of  cohesion  and  of  gravity  ;  it  there- 
fore exerts  its  power  against  both  the  attraction  of  gravitation  and  the  at- 
traction of  cohesion.  But  as  the  attraction  of  cohesion  does  not  exist  in 
aeriform  fluids,  the  expansive  power  of  heat  upon  them  has  nothing  to 
contend  with  but  gravity.  Any  increase  of  temperature,  therefore,  ex- 
pands an  elastic  fluid  prodigiously,  and  a  diminution  of  heat  condenses  it. 


141.  What  is  the  weight  of  a  column  of  air  one  inch  square  at  the  base, 
and  reaching  to  the  top  of  the  atmosphere  ?  Is  the  pressure  exerted  equally 
in  all  directions  ? 

142.  What  is  meant  by  the  elasticity  of  the  air?  How  do  the  aeriform 
fluids  differ  from  liquids ?  When  is  the  air  said  to  be  rarefied?  When 
condensed  ?    Is  the  air,  near  the  surface  of  the  earth,  rare  or  dense  ? 


1^00  NATURAL  PHILOSOPHY. 

143.  Air  becomes  a  mechanical  agent  by  nneans  of  its 
weight,  its  elasticity,  its  inertia,  and  its  fluidity.* 

144.  A  vacuum  is  a  space  from  which  air  and  every 
other  substance  has  been  removed. 

The  Torricellian  vacuum  was  discovered  by  Torricelli,  and 
was  obtained  in  the  following  manner.  A  tube  closed  at 
one  end,  and  about  32  inches  long,  was  filled  with  mercury  ; 
the  open  end  was  then  covered  with  the  finger  so  as  to  pre- 
vent the  escape  of  the  mercury,  and  the  tube  inverted  and 
plunged  into  a  vessel  of  mercury  ;  the  finger  was  then  removed 
and  the  mercury  permitted  to  run  out  of  the  tube.  It  was 
found,  however,  that  the  mercury  still  remained  in  the  tube  to 
the  height  of  about  thirty  inches,  leavmg  a  vacuum  at  the  top 
of  aboul  two  inches.  This  vacuum,  called  from  the  discoverer 
the  Torricelhan  vacuum,  is  the  most  perfect  that  has  been  dis- 
covered.f 

»  The  fiuidity  of  air  invests  it,  as  it  invests  all  other  liquids,  with  the 
power  of  transmitting  pressure.  But  it  has  already  been  shown,  under 
the  head  of  Hydrostatics,  that  fluidity  is  a  necessary  consequence  of  the  in- 
dependent gravitation  of  the  particles  of  a  fluid.  It  may,  therefore,  be 
included  among  the  efifects  of  weight. 

The  inertia  of  air  is  exhibited  in  the  resistance  which  it  opposes  to  mo- 
tion, which  has  already  been  noticed  under  the  head  of  Mechanics  This 
is  clearly  seen  in  its  eflect  upon  falling  bodies,  as  will  be  exemplified  m  the 
experiments  with  the  air-pump. 

t  As  this  is  one  of  the  most  important  discoveries  of  the  science  ot 
Pneumatics,  it  is  thought  to  be  deserving  of  a  labored  explanation.  The 
whole  phenomenon  is  the  result  of  the  equilibrium  of  fluids.  The  atmo- 
sphere pressing  by  its  weight  (15  pounds  ou  every  square  mch)  on  the  sur- 
face of  the  mercury  in  the  vessel,  counterpoised  the  column  of  mercury  m 
the  tube  when  it  was  about  30  inches  high,  showing  thereby  that  a  column 
of  the  atmosphere  is  equal  in  weight  to  a  column  of  mercury  of  the  sarne 
base,  having  a  height  of  30  inches.  Any  increase  or  dimmution  in  the 
density  of  the  air  produces  a  corresponding  alteration  in  its  weight,  and  con- 
sequently, in  its  ability  to  sustain  a  longer  or  a  shorter  column  of  mercury. 
Had  water  been  used  instead  of  mercury,  it  would  have  required  a  height 
of  about  33  feet  to  counterpoise  the  weight  of  the  atmospheric  column. 
Other  fluids  may  be  used,  but  the  perpendicular  height  of  the  column  of 
any  fluid,  to  counterpoise  the  weight  of  the  atmosphere,  must  be  as  much 


143.  How  does  the  air  become  a  mechanical  agent  1 

144.  What  is  a  vacuum  ? 


PNEUMATICS. 


101 


145.  The  barometer  is  an  instrument  to  measm'e  the 
weight  of  the  atmosj)here,  and  thereby  to  indicate  the 
variations  of  the  weather.* 

1.  Fig.  69  represents  a  barometer.  It  consists  of  a  long 
glass  tube,  about  thirty-three  inclies  in  length, 
closed  at  the  upper  end  and  filled  with  mercury. 
The  tube  is  then  inverted  in  a  cup,  or  leather  bag, 
of  mercury,  on  which  the  pressure  of  the  atmo- 
sphere is  exerted.  As  the  tube  is  closed  at  the 
top,  it  is  evident  that  the  mercury  cannot  de- 
scend in  the  tube  without  producing  a  vacuum. 
The  pressure  of  the  atmosphere  (which  is  capable 
of  supporting  a  column  of  mercury  of  about  30 
inches  in  height)  prevents  the  descent  of  the 
mercury ;  and  the  instrument,  thus  constructed, 
becomes  an  implement  for  ascertaining  the  weight 
of  the  atmosphere.  As  the  air  varies  in  weight 
or  pressure,  it  must,  of  course,  influence  the  mer- 
cury in  the  tube,  which  will  rise  or  fall  in  exact  proportion  with 
the  pressures  When  the  air  is  thin  and  light,  the  pressure  is 
less,  and  the  mercury  will  descend  ;  and  when  the  air  is  dense 
and  heavy,  the  mercury  will  rise.  At  the  side  of  the  tube 
there  is  a  scale,  marked  inches  and  tenths  of  an  inch,  to  note 
the  rise  and  fall  of  the  mercury.f 

g^reater  than  that  of  mercury  as  the  specific  gravity  of  mercury  exceeds 
that  of  the  fluid  employed. 

*  The  elasticity  of  the  air  causes  an  increase  or  diminution  of  its  bulk, 
according  as  it  is  afFected  by  heat  and  cold  ;  and  this  increase  and  diminu- 
tion of  bulk  materially  affect  its  specific  gravity.  The  height  of  a  column 
of  mercury  that  can  be  sustained  by  a  column  of  the  atmosphere  must, 
therefore,  be  affected  by  the  state  of  the  atmosphere.  The  instrument 
used  to  indicate  these  changes  is  called  a  barometer,  from  two  Greek 
words  signifying  a  measure  of  the  weight,  that  is,  of  the  atmosphere.  A 
Thermometer  is  a  measure  of  the  heat,  and  a  Hygrometer  a  measure  of 
the  moisture  of  the  atmosphere. 

t  Any  other  fluid  may  be  used  as  well  as  mercury,  provided  the  length 

145.  What  is  a  barometer?  What  does  the  word  barometer  mean? 
What  is  a  thermometer  ?  What  does  the  word  thermometer  mean  ?  What 
is  a  hygrometer?  What  does  the  word  hygrometer  mean?  What  figure 
represents  a  barometer  ?  Explain  its  construction.  What  height  of  mer- 
cury is  the  pressure  of  the  atmosphere  capable  of  sustaining  ?  What  effect 
has  the  pressure  of  the  atmosphere  on  the  mercury  in  the  tube  ? 


102 


NATURAL  PHILOSOPHY. 


2.  The  pressure  of  the  atmosphere  on  the  mercury,  in  the  bag 
or  cup  of  a  barometer,  bemg  exerted  on  the  principle  of  the 
eqiiihbrium  of  fluids,  must  vary  according  to  the  situation  in 
which  the  barometer  is  placed.  For  this  reason  it  will  be  the 
greatest  in  valleys  and  low  situations,  and  least  on  the  top  of 
high  mountains.  Hence  the  barometer  is  often  used  to  ascer- 
tain the  height  of  mountains  and  other  places  above  the  level 
of  the  sea.* 

of  the  tube  be  extended  in  proportion  to  the  specific  gravity  of  the  fluid 
Thus,  a  tube  filled  with  water  must  bo  33  feet  long,  because  the  atmo- 
sphere will  support  a  column  of  water  of  that  height.  Mercury  is  used, 
therefore,  in  the  construction  of  the  barometer,  because  it  does  not  require 
so  long  a  tube  as  any  other  fluid.  It  may  here  bo  remarked,  that  the  ail 
is  the  heaviest  in  dry  weather,  and  that,  consequently,  the  mercury  will  then 
rise  highest.  In  wet  weather  the  dampness  renders  the  air  less  salubrious, 
and  it  appears,  therefore,  more  heavy  then,  although  it  is,  in  fact,  much 
lighter.  The  greatest  depression  of  the  barometer  occurs  daily  at  about 
4  o'clock,  both  in  the  morning  and  in  the  afternoon,  and  its  highest  eleva- 
tion at  about  10  o'clock  morning  and  night.  In  summer,  these  extreme 
points  are  reached  an  hour  or  two  earlier  in  the  morning,  and  as  much 
later  in  the  afternoon. 

*  As  the  air  diminishes  in  density,  upwards,  it  follows,  that  it  must  be 
more  rare  upon  a  hill  than  on  a  plain.  In  very  elevated  situations  it  is 
so  rare  that  it  is  scarcely  fit  for  respiration,  or  breathing  ;  and  the  expan- 
sion which  takes  place  in  the  more  dense  air  contained  within  the  body  is 
often  painful :  it  occasions  distention,  and  sometimes  causes  the  bursting 
of  the  smaller  blood-vessels,  in  the  nose  and  ears.  Besides,  in  such  situa- 
tions, we  are  more  exposed  botli  to  heat  and  cold ;  for,  though  the  at- 
mosphere is  itself  transparent,  its  lower  regions  abound  with  vapors  and 
exhalations  from  the  earth,  which  float  in  it,  and  act  in  some  degree  as  a 
covering,  which  preserves  us  equally  from  the  intensity  of  the  sun's  rays, 
and  from  the  severity  of  the  cold. 


In  what  proportion  does  the  mercury  rise  and  fall  ?  In  what  way  can 
barometers  be  made  of  other  fluids?  Why  is  mercury  used  in  preference 
to  any  other  fluid?  Is  the  air  heaviest  in  wet  or  dry  weather?  On  what 
principle  is  the  pressure  of  the  atmosphere  on  the  mercury,  in  the  cup  of 
a  barometer,  exerted?  What  follows  from  this?  For  what  other  purpose, 
besides  measuring  the  pressure  of  the  atmosphere,  and  foretelling  the  varia- 
tions of  the  weather,  is  the  barometer  used  ?  Is  the  air  the  more  dense  at 
the  surface  of  the  earth  or  upon  a  hill?  What  is  a  thermometer?  What 
figure  represents  a  thermometer?    Explain  its  construction. 


I'NEUMATICS. 


103 


3.  The  thermometer  is  an  inst*-ument  used  to  's- 
indicate  the  temperature  of  the  atmosphere.  In 
appearance  it  resembles  a  barometer,  but  it  is 
constructed  on  a  difibrent  principle,  and  for  a 
different  purpose.  It  consists  of  a  capillary 
tube,  closed  at  the  top,  and  terminated  with 
a  bulb,  which  is  filled  with  mercury.*  As  heat 
expands  and  cold  contracts  most  substances,  it 
follows,  that  in  warm  weather  the  mercury  must 
be  expanded  and  will  rise  in  the  tube,  and  that 
in  cold  weather  it  will  contract  and  sink.  Hence 
the  instrument  becomes  a  correct  measure  for  the 
heat  and  cold  of  the  air.  A  scalef  is  placed  at 
the  side  of  the  tube,  to  mark  the  degree  of  heat 
or  cold,  as  it  is  indicated  by  the  rise  and  fall  of 
the  mercury  in  the  capillary  tube. 

4  The  hygrometer,  for  measuring  the  degree 
of  moisture  in  the  air,+  may  be  constructed  ot 
any  thing  which  contracts  and  expands  ^7  the  moisture  or 
dryness  of  the  atmosphere,  such  as  most  kinds  of  wood  ,  cat- 
gut, twisted  cord  the  beard  of  wild  oats,  &c. 

*  Any  other  liquid  which  is  expanded  by  heat  and  contracted  by  cold, 
such  as  spirits  of  wine,  &c.,  will  answer  instead  of  mercury. 

t  There  are  several  different  scales  applied  to  the  thermometer,  of 
which  those  of  Fahrenheit,  Reaumur,  Delisle,  and  Celsius  are  the  princi- 
pal The  tliermometer  in  common  use  in  this  country,  is  graduated  by 
Fahrenheit's  scale,  which,  commencing  with  0,  or  zero,  extends  upwards 
to  212  degrees,  the  boiling  point  of  water,  and  downwards  to  20  or  M  de- 
grees. The  scales  of  Reaumur  and  Celsius  fix  zero  at  the  freezing  point 
of  water  ;  and  that  of  Delisle  at  the  boiling  point. 

t  Bv  the  action  of  the  sun's  heat  upon  the  surface  of  the  earth,  whether 
land  or  water,  immense  quantities  of  vapor  are  raised  into  the  atmosphere, 
supplying  materials  for  all  the  water  which  is  deposited  again  m  the  va- 
rious'forms  of  dew,  fog,  rain,  snow,  and  hail.  Experiments  have  been 
made  to  show  the  quantity  of  moisture  thus  raised  from  the  ground  by  the 
heat  of  the  sun.  Dr.  Watson  found  that  an  acre  of  ground,  apparently 
dry,  and  burnt  up  by  the  sun,  dispersed  into  the  air  sixteen  hundred  gal- 


What  effect  have  heat  and  cold  on  most  substances?    What  follows 
from  this  1    Whose  scale  is  generally  used  in  this  country  ?    For  what  is 
the  hyo-rometer  used  ?    Of  what  kind  of  substances  may  it  be  constructed 
What  experiment  is  given  in  the  note  to  show  the  quantity  of  moisture 
raised  from  the  ground  by  the  heat  of  the  sun? 


4 


104 


NATURAL  PHILOSOPHY. 


146.  The  impenetrability  of  air  prevents  the  ascent 
of  water  into  any  inverted  vessel,  unless  the  air  is  first 
permitted  to  escape. 

1.  If  a  tube,  closed  at  one  end,  or  an  inverted  tumbler,  be 
inserted  at  its  open  end,  in  a  vessel  of  water,  the  water  will 
not  rise  in  the  tube  or  tumbler,  to  a  level  with  the  water  in 
the  vessel,  on  account  of  the  impenetrability  of  the  air  within 
the  tube.  But  if  the  tube  be  open  at  both  ends,  the  water  will 
rise,  because  the  air  can  escape  through  the  upper  end.  It  is 
on  this  principle  that  the  diving-bell  (or  the  diver's  bell,  as  it 
is  sometimes  called)  is  constructed. 

2.  Fig.  71  represents  a  diving-bell.  It  consists  of  a  large 
heavy  vessel,  formed  hke  a  bell,  (but  may  be  made  of  any  other 
shape,)  with  the  mouth  open.    It  descends  into  the  water  with 

Ions  of  water  in  the  space  of  twelve  hours.  His  experiment  was  thus 
made  :  he  put  a  glass,  mouth  downwards,  on  a  grass-plot,  on  which  it  had 
not  rained  for  above  a  month.  In  less  than  two  minutes  the  inside  was 
covered  with  vapor;  and  in  half  an  hour  drops  began  to  trickle  down  its 
inside.  The  mouth  of  the  glass  was  20  square  inches.  There  are  1296 
square  inches  in  a  square  yard,  and  4840  square  yards  in  an  acre.  When 
the  glass  had  stood  a  quarter  of  an  hour,  he  wiped  it  with  a  piece  oi 
muslin,  the  weight  of  which  had  been  previously  ascertained.  When 
the  glass  had  been  wiped  dry,  he  again  weighed  the  muslin,  and  found 
that  its  weight  had  been  increased  six  grains  by  the  water  collected  from 
20  square  inches  of  earth;  a  quantity  equal  to  1600  gallons,  from  an  acre, 
in  12  hours.  Another  experiment,  after  rain  had  fallen,  gave  a  much 
larger  quantity.    (See  No.  9.) 

When  the  atmosphere  is  colder  than  the  earth,  the  vapor,  which  arises 
from  the  ground,  or  a  body  of  water,  is  condensed  and  becomes  visible 
This  is  the  way  that  fog  is  produced.  When  the  earth  is  colder  than  the 
atmosphere,  the  moisture  in  the  atmosphere  condenses  in  the  form  of  dew, 
on  the  ground,  or  other  surfaces. 

Clouds  are  nothing  more  than  vapor,  condensed  by  the  cold  of  the  upper 
regions  of  the  atmosphere. 

Rain  is  produced  by  the  sudden  cooling  of  large  quantities  of  watery 
vapor. 

Snow  and  hail  are  produced  in  a  similar  manner,  and  differ  from  rain 
only  in  the  degree  of  cold  which  produces  them. 


146.  Is  air  impenetrable,  like  other  substances  ?  -  How  is  this  shown  ? 
Upon  what  principle  is  the  diving-bell  constructed  ?  What  figure  repre- 
sents the  diving-bell?  Why  does  not  the  water  rise  in  the  bell?  Explain 
the  figure. 


PNEUMATICS. 


105 


its  mouth  downwards.    The  air  within  Fig.  71. 

it  having  no  outlet  is  compelled,  by  the 

order  of  specific  gravities,  to  ascend  in 

the  bell,  and  thus  (as  water  and  air 

cannot  occupy  the  same  space  at  the 

same  time)  prevents  the  water  from 

rising  in  the  bell.    A  person,  therefore, 

may  descend  with  safety  in  the  bell  to 

a  great  depth  in  the  sea,  and  thus  re- 
cover valuable  articles  that  have  been 
lost.  A  constant  supply  of  fresh  air  is 
sent  down,  either  by  means  of  barrels, 
or  by  a  forcing-pump.  In  the  Fig.,  _B 
represents  the  bell  with  the  diver  in  it. 
C  is  a  bent  metallic  tube  attached  to 
one  side  and  reaching  the  air  within ; 
and  P  is  the  forcing-pump  through 
which  air  is  forced  into  the  bell.  The 
forcing-pump  is  attached  to  the  tube 
by  a  joint  at  D.  When  the  bell  de- 
scends to  a  great  depth,  the  pressure 

of  the  water  condenses  the  air  within  the  bell,  and  causes  the 
water  to  ascend  in  the  bell.  This  is  forced  out  by  constant 
accessions  of  fresh  air,  supphed  as  above  mentioned.  Great 
care  must  be  taken  that  a  constant  supply  of  fresh  air  is  sent 
down,  otherwise  the  lives  of  those  within  the  bell  will  be  en- 
dangered. The  heated  and  impure  air  is  allowed  to  escape 
through  a  stop-cock  in  the  upper  part  of  the  bell. 

147.  Water  is  raised  in  the  common  pump  by  means 
of  the  pressure  of  the  atmosphere  on  the  surface  of  the 
water.  A  vacuum  being  produced  by  raising  the  piston 
or  pump-box  *  the  water  below  is  forced  up  by  the  at- 
mospheric pressure,  on  the  principle  of  the  equilibrium 

1^  In  order  to  produce  such  a  vacuum,  it  is  necessary  that  the  piston  or 
box  should  be  accuratelv  fitted  to  the  bore  of  the  pump;  for  if  the  air 
above  the  piston  has  any  means  of  rushing  in  to  fill  the  vacuum,  as  it  is 
produced  by  the  raising  of  the  piston,  the  water  will  not  ascend.  The  pis- 
ton is  generally  worked  by  a  lever,  which  is  the  handle  of  the  pump,  not 
represented  in  the  figure. 


147.  By  what  means  is  water  raised  in  the  common  pump?  How  is 
the  pressure  removed? 


i06 


NATURAL  PHILOSOPHY. 


of  fluids.  On  this  principle  the  water  can  be  raised 
only  to  the  height  of  about  thirty-three  feet,  because 
the  pressure  of  the  atmosphere  will  sustain  a  column  of 
water  of  that  height  only. 

Fig.  'Z  2  represents  the  common  pump,  called  ^^s-  72 
the  sucking-pump.  The  body  consists  of  a  large  r-Q^ 
tube,  or  pipe,  the  lower  end  of  which  is  to  be  1 1 
immersed  in  the  water  which  it  is  designed  to 
raise.  P  is  the  piston,  Y  a  valve*  in  the  piston, 
which,  opening  upv/ards,  admits  the  water  to 
rise  through  it,  but  prevents  its  return.  Y  is  a 
similar  valve  in  the  body  of  the  pump,  below  the 
piston.  When  the  pump  is  not  in  action  the 
valves  are  closed  by  their  own  weight ;  but  when 
the  piston  is  raised  it  draws  up  the  column  of 
water  which  rested  upon  it,  producing  a  vacuum 
between  the  piston  and  the  lower  valve  Y.  The 
water  below,  immediately  rushes  through  the 
lower  valve,  and  fills  the  vacuum.  When  the 
piston  descends  a  second  time,  the  water  in  the 
body  of  the  pump  passes  through  the  valve  Y, 
and  on  the  ascent  of  the  piston  is  lifted  up  by 

the  piston,  and  a  vacuum  is  again  formed  below,  f  

which  is  immediately  filled  by  the  water  rushing 
through  the  lower  valve  Y.    In  this  manner  the  body  of  the 
pump  is  filled  with  water,  until  it  reaches  the  spout  S,  where 
it  runs  out  in  an  interrupted  stream. f 

*  A  valve  is  a  lid  or  cover,  so  contrived  as  to  open  a  communication  in 
one  way  and  close  it  in  the  other.  Valves  are  made  in  different  ways,  ac- 
cording to  the  use  for  which  they  are  intended.  In  the  common  pump, 
they  are  generally  made  of  thick  leather  partly  covered  with  wood.  In 
the  air-pump  they  are  made  of  oiled  silk,  or  thin  leather  softened  with 
oil.  The  clapper  of  a  pair  of  bellows  is  a  familiar  specimen  of  a  valve. 
The  valves  of  a  pump  are  commonly  called  boxes. 

t  Although  water  can  be  raised  by  tlie  atmospheric  pressure  only  to  the 
height  of  33  feet  above  the  surface,  the  common  pump  is  so  constructed, 
that  after  the  pressure  of  the  atmosphere  has  forced  the  water  through  the 


What  figure  represents  the  common  pump  ?  Explain  it.  Which  of  the 
mechanical  powers  is  the  handle  of  the  pump?  How  high  can  water  be 
raised  by  the  common  pump?  Why?  Why  is  the  common  pump  some- 
times called  the  lifting-pump? 


FNEUMATICS. 


107 


148.  The  forcing-pump  differs  from  the  common 
pump  in  having  a  "forcing  power  added  to  raise  the 
water  to  any  desired  height. 

1.  Fig.  Y3  represents  the  forcing- 
pump.  The  body  and  lower  valve 
y  are  similar  to  those  in  the  com-  ^ 
mon  pump.  The  piston  P  has  no 
valve,  but  is  sohd  ;  when,  therefore, 
the  vacuum  is  produced  above  the 
lower  valve,  the  water  on  the  de- 
scent of  the  piston  is  forced  through 
the  tube  into  the  reservoir  or  air- 
vessel  R,  where  it  compresses  the 
air  above  it.  The  air,  by  its  elas- 
ticity, forces  the  water  out  through 
the  jet  J  in  a  continued  stream  and 
with  great  force.  It  is  on  this  prin- 
ciple that  fire-engines  are  constructed. 

2.  Sometimes  a  pipe  with  a  valve  in  it  is  substituted  for  the 
air-vessel ;  the  water  is  then  thrown  out  in  a  continued  stream, 
but  not  with  so  much  force. 

149.  Wind  is  air  put  in  motion.* 

When  any  portion  of  the  atmosphere  is  heated,  it  becomes 
rarefied,  its  specific  gravity  is  diminished,  and  it  consequently 
rises.  The  adjacent  portions  immediately  rush  into  its  place 
to  restore  the  equilibrium.  This  motion  produces  a  current 
which  rushes  into  the  rarefied  spot  from  all  directions.  This  is 
what  we  call  wind.    The  portions  north  of  the  rarefied  spot, 

valve  in  the  body  of  the  pump,  and  the  descent  of  the  piston  has  forced  it 
through  the  valve  in  the  piston,  it  is  lifted  up,  when  the  piston  is  raised. 
For  this  reason,  this  pump  is  sometimes  called  the  lifting  pump.  The 
distance  of  the  lower  valve  from  the  surface  of  the  water  must  never  ex- 
ceed 32  feet ;  and  in  practice  it  must  be  much  less. 

*  There  are  two  ways  in  which  the  motion  of  the  air  may  arise.  It 
may  be  considered  as  an  absolute  motion  of  the  air,  rarefied  by  heat  and 
condensed  by  cold ;  or  it  may  be  only  an  apparent  motion,  caused  by  the 
superior  velocity  of  the  earth  in  its  daily  revolution. 

148.  How  does  the  forcing-pump  differ  from  the  common  pump?  What 
figure  represents  the  forcing-pump?    Explain  it. 

149.  What  is  wind?  In  what  two  ways  may  the  motion  of  the  air  be 
explained?    Explain  the  manner  in  which  the  air  is  put  in  motion. 


108 


NATURAL  PHILOSOPHY. 


produce  a  north  wind ;  those  to  the  south  produce  a  south 
wind ;  while  those  to  the  east  and  west,  in  hke  manner,  form 
currents  moving  in  opposite  directions.  At  the  rarefied  spot, 
agitated  as  it  is  by  winds  from  all  directions,  turbulent  and 
boisterous  weather,  whirlwinds,  hurricanes,  rain,  thunder  and 
lightning  prevail.  This  kind  of  weather  occurs  most  frequent- 
ly in  the  torrid  zone,  where  the  heat  is  greatest.  The  air  be- 
ing more  rarefied  there  than  in  any  other  part  of  the  globe,  is 
lighter,  and,  consequently,  ascends :  that  about  the  polai 
regions  is  continually  flowing  from  the  poles  to  the  equator,  to 
restore  the  equilibrium ;  while  the  air  rising  from  the  equatoi 
flows  in  an  upper  current  towards  the  poles,  so  that  the  polar 
regions  may  not  be  exhausted.^    A  regular  east  wind  prevail? 

*  From  what  has  now  been  said,  it  appears  that  there  is  a  circulation 
of  air  in  the  atmosphere  ;  the  air  in  the  lower  strata  flowing  from  tht3 
poles  to  the  equator,  and  in  the  upper  strata  flowing  back  from  the  equator 
to  the  poles.  It  may  here  be  remarked,  that  the  periodical  winds  are 
more  regular  at  sea  than  on  the  land ;  and  the  reason  of  this  is,  that  the 
land  reflects  into  the  atmosphere  a  much  greater  quantity  of  the  sun's  rays 
than  the  water  ;  therefore,  that  part  of  the  atmosphere  which  is  over  the 
land  is  more  heated  and  rarefied  than  that  which  is  over  the  sea.  This 
occasions  the  wind  to  set  in  upon  the  land,  as  we  find  it  regularly  does  on 
the  coast  of  Guinea  and  other  countries  in  the  torrid  zone.  There  are  cer- 
tain winds  called  trade-winds,  the  theory  of  which  may  be  easily  explained 
on  the  principle  of  rarefaction,  affected  as  it  is  by  the  relative  position  of 
the  different  parts  of  the  earth  with  the  sun,  at  ditTerent  seasons  of  tlie 
year  and  at  various  parts  of  the  day.  A  knowledge  of  the  laws  by  which 
these  winds  are  controlled,  is  of  importance  to  the  mariner.  When  the 
place  of  the  sun,  with  respect  to  the  different  positions  of  the  earth  at 
the  diff*erent  seasons  of  the  year  is  understood,  it  will  be  seen  that  they  all 
depend  upon  the  same  principle.  The  reason  that  the  wind  generally  sub- 
sides at  the  going  down  of  the  sun,  is,  that  the  rarefaction  of  the  air,  in 
the  particular  spot  which  produces  the  wind,  diminishes  as  the  sun  de- 
clines, and  consequently,  the  force  of  the  wind  abates.  The  great  variety 
of  winds  in  the  temperate  zone  is  thus  explained.  The  air  is  an  exceed- 
ingly elastic  fluid,  yielding  to  the  slightest  pressure  ;  the  agitations  in  it, 
therefore,  caused  by  the  regular  winds,  whose  causes  have  been  explained, 
must  extend  every  way  to  a  great  distance ;  and  the  air,  therefore,  in  all 
climates  will  suflfer  more  or  less  perturbation,  according  to  the  situation  of 
the  country,  the  position  of  mountains,  valleys,  and  a  variety  of  other 
causes.    Hence  every  climate  must  be  liable  to  variable  winds.  The 


How  are  the  north,  south,  east,  and  west  winds  produced  ? 


about  the  (Mjualoi",  caused  1)\'  (lie  rai'c  fact  ion  of  tlu;  aii'  [)n)- 
(luct'il  l>y  (lie  sun  in  his  daiK'  coui'sc  from  cast  (o  wcsl.  'J'his 
wiiuL  I'oinhiniiiL;"  \\ilh  llial,  iVoni  (he  poh's,  caus(!s  ji  conslanl 
noi  lluast  wind,  for  about  tliirty  dogrees  north  of  the  etjuator, 
and  ;i  soutlioast  wind  at  the  same  distance  south  of  tlie 
equator. 

OF  THE  AIR-PUMP. 

150.  The  air-pump  is  an  instrument  for  exhaustinf^ 
the  air  from  a  vessel  prepared  for  the  purpose.  This 
vessel  is  called  a  receiver,  and  is  made  of  glass  in  order 
that  the  etiects  of  the  removal  of  the  air  may  be  seen. 

x\ir-pumps  are  made  in  a  great  variety  of  forms ; 
but  all  are  constructed  on  the  principle,  that  when  any 
portion  of  confined  air  is  removed,  the  residue  immedi- 
ately expanding,  by  its  elasticity  fills  the  space  occupied 
by  the  portion  that  has  been  withdrav^n. 

1.  From  this  statement  it  will  appear  that  a  perfect  vacuum 
can  never  be  obtained  by  the  air-pump  as  at  present  construct- 
ed. But  so  much  of  the  air  within  a  receiver  may  be  exhausted 
that  the  residue  will  be  reduced  to  such  a  degree  of  rarity 
as  to  subserve  most  of  the  practical  purposes  of  a  vacuum. 
Fig.  74  represents  a  single  barrel  air-pump,  used  both  for 
condensing  and  exhausting.  A  D  is  the  stand  or  platform 
of  the  instrument,  which  is  screwed  down  to  the  table  by 

quality  of  winds  is  affected  by  the  countries  over  which  they  pass;  and 
they  are  sometimes  rendered  pestilential  by  the  heat  of  deserts,  or  the  pu- 
trid exhalations  of  marshes  and  lakes.  Thus,  from  the  deserts  of  Africa, 
Arabia,  and  the  neighboring  countries,  a  hot  wind  blows,  called  Samiel, 
01  Simoom,  which  sometimes  produces  instant  death.  A  similar  wind 
blows  from  the  desert  of  Sahara,  upon  the  western  coast  of  Africa,  called 
the  Harmattan,  producing  a  dryness  and  heat  which  is  almost  insupport- 
able, scorching  like  the  blasts  of  a  furnace 


160.  What  is  an  air-pump?  What  is  *,he  vessel  called  from  which  the 
air  is  exhausted  ?  On  what  principle  are  all  air-pumps  constructed  ?  Can 
a  perfect  vacuum  ever  be  obtained?  Describe  the  air-pump  represented 
bv  Fig.  74 


110 


NATURAL  PHILOSOPHY. 


IICCD 


Fig  74 


means  of  a  clamp,  under- 
neath, which  is  not  repre- 
sented in  the  figure.  K  is 
the  glass  vessel  or  bulbed 
receiver  from  which  the 
air  is  to  be  exhausted.  P 
is  a  solid  piston,  accurate- 
ly fitted  to  the  bore  of 
the  cylinder,  and  H  the 
handle  by  which  it  is 
moved.  The  dotted  line 
T,  represents  the  commu- 
nication between  the  re- 
ceiver R  and  the  barrel 
B ;  it  is  a  tube  through 
which  the  air,  entering 

at  the  opening  I,  on  the  plate  of  the  pump,  passes  into  the 
barrel,  through  the  exhausting  valve  e  v.  c  v  is  the  conden- 
sing valve,  communicating  with  the  barrel  B  by  means  of  an 
aperture  near  e,  and  opening  outwards  through  the  condensing 
pipe  'p. 

2.  The  operation  of  the  pump  is  as  follows :  The  piston  P  being 
drawn  upwards  by  the  handle  H,  the  air  in  the  receiver  R,  ex- 
panding by  its  elasticity,  passes  by  the  aperture  I  through  the 
tube  T,  and  through  the  exhausting  valve  e  v  into  the  barrel. 
On  the  descent  of  the  piston,  the  air  cannot  return  through 
that  valve,  because  the  valve  opens  upwards  only :  it  must, 
therefore,  pass  through  the  aperture,  by  the  side  of  the  valve, 
and  through  the  condensing  valve  c  v  into  the  pipe  p,  where 
it  passes  out  into  the  open  air.  It  cannot  return  through  the 
condensing  valve  c  v,  because  that  valve  opens  outwards  only. 
By  continuing  this  operation,  every  ascent  and  descent  of  the 
piston  P  must  render  the  air  within  the  receiver  R  more  and 
more  rare,  until  its  elastic  power  is  exhausted.  The  receiver 
is  then  said  to  be  exhausted ;  and  although  it  still  contains  a 
small  quantity  of  air,  yet  it  is  in  so  rare  a  state  that  the  space 
within  the  receiver  is  considered  a  vacuum.'^ 

3.  From  the  explanation  which  has  been  given  of  the  opera- 
tion *of  this  air-pump,  it  will  readily  be  seen  that,  by  removing 
the  receiver  R,  and  screwing  any  vessel  to  the  pipe  p,  the  air 


*  The  only  known  method  of  procuring  a  perfect  vacuum,  is  that  pur- 
sued by  Torricelli,  which  has  already  been  explained. 


Ill 


m:\y  ])('  condcnscil  in  (he  vessel.  Thus  (he  j)Ui)ij)  is  in;i(l(i  to 
i'\h;iiisl  or  to  (•( )iHhMise.  wiiIkmiI  aher.'iliou.'^' 

t.  'riie  (h)iible  ait-piiin j)  (iilleis  tVoin  (he  sini;le  :iii-])um p,  in 
havini;-  two  barrels  aiul  two  })ist()iis  ;  which,  iiistciJid  of  beiiii;- 
moved  by  the  hand,  aie  worked  by  iiieiins  of  a  toothed  wiui(;l, 
})lavini;-  in  notches  of  the  jMston  rods. 

5.  Fio-.  75  repn^sents  Wio-htinan's  patent  levor  air-})umj), 
belonn-ino-  lo  "  ///c  Jiosto))  School  Set.'"    This  instrument  is  of 


Fig.  75. 


an  improved  construction,  and  differs  from  others  in  the  facih- 
ty  with  which  it  is  worked.    In  this  pump  the  piston  is  sta- 

*  Air-pumps  in  general  are  not  adapted  for  condensation  ;  that  office 
being  performed  by  an  instrument  called  "  a  condensing  syringe,^^  which 
is  an  air-pump  recersed,  its  valves  being  so  arranged  as  to  force  air  into 
a  chamber,  instead  of  drawing  it  out.  For  this  purpose,  the  valves  open 
inwards  in  respect  to  the  chamber,  while  in  air-pumps  they  open  outwards. 

A  gauge,  constructed  on  the  principle  of  the  barometer,  is  sometimes 
adjusted  to  the  air-pump  for  the  purpose  of  exhibiting  the  degree  of  ex- 
haustion 


112 


NATURAL  PHILOSOPHY. 


tionaiy,  while  motion  is  given  to  the  barrel  by  means  of  the 
lever  H.  The  barrel  is  kept  in  a  proper  position  by  means  of 
polished  steel  guides.^ 

151.  By  means  of  the  air-pump,  many  interesting 
experiments  may  be  performed,  illustrating  the  gravity, 
elasticity,  fluidity,  and  inertia  of  air. 


Fig.  76. 


EXPERIMENTS  ILLUSTRATING  THE  GRAVITY  OF  AIR. 

1.  Having  adjusted  the  receiver  to  the  plate  of  the  air- 
pump,  exhaust  the  air  and  the  receiver  will  be  held  firmly  on 
the  plate.  The  force  which  confines  it,  is  nothing  more  than 
the  weight  of  the  external  air,  which,  having  no  internal  pres- 
sure to  contend  with,  presses  Avith  a  force  of  nearly  1 5  pounds 
on  every  square  inch  of  the  external  surface  of  the  receiver. 

N.  B.  The  exact  amount  of  pressure  depends  on  the  degree 
of  exhaustion,  being  at  its  maximum  of  15  pounds  when  there 
is  a  peifect  vacuum.  On  readmitting  the  air  the  receiver  may 
be  readily  removed. 

2.  The  Magdeburgh  Cups  or  Hemispheres.  Fig.  76  repre- 
sents the  Magdeburgh  cups  or  hemispheres.  They  consist  of  two 
hollow  brass  cups,  the  edges  of  which  are 

accurately  fitted  together.  They  each  have 
a  handle,  to  one  of  which  a  stop- cock  is 
fitted.  The  stop-cock,  being  attached  to 
one  of  the  cups,  is  to  be  screwed  to  the 
plate  of  the  air-pump,  and  left  open. 
Having  joined  the  other  cup  to  that  on 
the  pump,  exhaust  the  air  from  Avithin 
them,  turn  the  stop-cock  to  prevent  its  re- 
admission,  and  screw  the  handle  that  had 
been  removed  to  the  stop- cock.  Two  per- 
sons may  then  attempt  to  draw  the  cups 
asunder.  It  will  be  found  that  great 
power  is  required  to  separate  them ;  but, 
on  readmitting  the  air  betAveen  them,  by 
turning  the  cock,  they  will  fall  asunder  by 

*  Mr.  Wighlman  has  published  a  small  volume,  entitled,  A  Con 
panioii  to  the  Air-pump,"  which  will  be  found  a  very  convenient  guide  fo 
the  management  of  the  pump,  and  the  skilful  performance  of  experiments 


151.  Give,  in  succession,  the  experiments  made  by  means  of  the  air 
pump.    Of  the  receiver     Of  the  Magdeburgh  cups. 


PNEUMATICS. 


113 


Fig.  77. 


Fig.  78. 


their  own  weight.  When  the  air  is  exhausted  from  withm 
them,  the  pressure  of  the  surrounding  air  upon  the  outside 
keeps  them  united.  This  pressure  being  equal  to  a  pressure 
of  lifteen  pounds  on  every  square  inch  of  the  surface,  it  toilows 
that  the  larger  the  cups  or  hemispheres  the  more  difhcult  it 
will  be  to  separate  them.^ 

3.  The  Hand-glass.  Fig.  77  is  nothing 
more  than  a  tumbler,  open  at  both  ends, 
with  the  top  and  bottom  ground  smooth,  so 
as  to  fit  the  brass  plate  of  the  air-pump. 
Placing  it  upon  the  plate,  cov.er  it  closely 
with  the  palm  of  the  hand,  and  work  the 
pump.  The  air  within  the  glass  being  thus 
exhausted,  the  hand  will  be  pressed  down 
by  the  weight  of  the  air  above  it :  on  read- 
mitting the  air,  the  .hand  may  be  easily  re- 
moved. 

4.  The  Bladder-glass.  Fig.  78  is  a  bell- 
shaped  glass,  covered  with  a  piece  of  bladder, 
which  is  tied  tightly  around  its  neck.  Thus 
prepared,  it  may  be  screwed  to  the  plate  of 
the  air-pump,  or  connected  with  it  by  means 
of  an  elastic  tube.  On  exhausting  the  air 
from  the  glass,  the  external  pressure  of  the 
air  on  the  bladder  will  burst  it  inwards  with 
a  loud  explosion. 

5.  The  India-rubber  Glass.  Fig.  79  is 
a  glass  similar  to  the  one  represented  in  the 
last  figure,  covered  with  india-rubber.  The 
same  experiments  may  be  made  with  this  as 
were  mentioned  in  the  last  article,  but  with 
difterent  results.  Instead  of  bursting,  the 
india-rubber  will  be  pressed  inwards  the  whole  depth  of  the 
glass. 

*  Otto  Giiericke,  the  inventor  of  the  air-pump,  prepared  two  hem- 
ispheres, two  feet  in  diameter,  and  having  accurately  fitted  them  together, 
and  exhausted  the  air,  30  horses  harnessed  to  them  were  unable  to  sep- 
arate  them.  When  more"  horses  were  added,  the  hemispheres  parted  with 
a  loud  report. 


Fig.  79. 


Explain  the  experiment  with  the  hand-glass.  Of  the  bladder-glass.  Of 
the  india-rubber  glass. 


114 


NATURAL  PHILOSOPHY. 


Fig.  81. 


6.  The  Fountain-Glass  and  Jet.  Fig.  80  represents  the 
jet,  which  is  a  small  brass  tube.  Fig.  81  is  the  fountain- 
glass.  The  expeiiment  with  these  in- 
struments is  designed  to  show  the 
pressure  of  the  atmosphere  on  the 
surface  of  liquids.  Screw  the  straight 
jet  to  the  stop-cock,  the  stop -cock  to 
the  fountain-glass,  with  the  straight  jet 
inside  of  the  fountain-glass,  and  the 
lower  end  of  the  stop-cock  to  the  plate 
of  the  air-pump,  and  then  open  the 
stop-cock.  Having^  exhausted  the  air 
from  the  fountain-glass,  close  the  stop- 
cock, remove  the  glass  from  the  pump,  and,  im- 
mersing it  in  a  vessel  of  water,  open  the  stop- 
cock. ^  The  pressure  of  the  air  on  the  surface  of  the  water  will 
cause  it  to  rush  up  into  the  glass  hke  a  fountain. 

7.  Pneumatic  Scales  for  weighing  Air.  Fig.  82  repre- 
sents the  flask  or  glass  vessel  and  scales  for  A^^io-hino-  air 
Weigh  the  flask  when  full  of  air :  then 

exhaust  the  air  and  weigh  the  flask 
again.  The  difference  between  its  pres- 
ent and  former  weight  is  the  weight 
of  the  air  that  was  contained  in  the 
flask. 

8.  The  Sucker.  A  circular  piece 
of  wet  leather,  with  a  string  attached 
to  the  centre,  being  pressed  upon  a 
smooth  surface,  will  adhere  with  con- 
siderable tenacity,  when  drawn  upwards  by  the  string.  The 
string  in  this  case  must  be  attached  to  the  leather  so^hat  no 
air  can  pass  under  the  leather. 

9.  The  Mercurial  or  Water  Tube.  Exhaust  the  air  from 
a  glass  tube  three  feet  long,  fitted  mth  a  stop-cock  at  one 
end,  and  then  immerse  it  in  a  vessel  containing  mercury  or 
water.  On  turning  the  stop-cock,  the  mercury  will  rise  to  the 
height  of  nearly  30  inches;  or,  if  immersed  in  water,  the 
water  will  rise  and  fill  the  tube,  and  would  fill  it  were  it  30 
feet  long.  This  experiment  shows  the  manner  in  which  water 
is  raised  to  the  boxes  or  valves  in  common  water-pumps. 


Explain  the  experiment  of  the  fountain-glass  and  jet.  Explain  the 
Dneumatic  scales  for  weighing  air.    Explain  the  sucker.    Mercurial  tube. 


PNEUMATICS. 


116 


EXPERIMENTS  SHOWING  THE  ELASTICITY  OF  AIR. 

1  Place  an  iiidia-mbber  bag,  or  a  bladder,  partly  inflated, 
and  tio-litly  closed,  under  the  receiver,  and  on  exhausting  the  an% 
the  air  within  the  bag  or  bladder  expanding,  fill  the  bag 
On  readmitting  the  air,  the  bag  will  collapse.  The  experimeirt 
may  also  be  made  wath  some  kinds  of  shrivelled  fruit,  it  tne 
skin  be  sound.  The  internal  air  expanding  v/ill  give  the  truit 
a  fresh  and  plump  appearance,  which  will  disappear  on  the  re- 
admission  of  the  air. 

2  The  same  principle  may  be  illustrated  by  the  mdia- 
rubber  and  bladder  glasses,  if  they  have  stop-cocks  to  conline 

the  air.  ....  i  •  o 

3  A  small  bladder  partly  filled  wath  air  may  be  sunk  m  a 
vessel  of  water  by  means  of  a  weight,  and  placed  under  the 
receiver.  On  exhausting  the  air  from  the  receiver,  the  air  m 
the  bladder  will  expand,  and  its  specific  gravity  bemg  thus 
diminished,  the  bladder  with  the  weight^  will  rise.  On  re- 
admittino'  the  air  the  bladder  will  sink  again. 

4  Air  contained  in  Water  and  in  Wood.  Place  a  vessel 
of  water  under  the  receiver,  and  on  exhaustmg  the  air  trom  the 
receiver,  the  air  in  the  water  previously  mvisible  will  make  its 
appearance  in  the  form  of  bubbles,  presenting  the  semblance 
of  ebullition.  .      .  j  •  xi 

5  A  piece  of  light  porous  wood  being  immersed  m  the 
water  below  the  surface,  the  air  will  be  seen  issumg  m  bub- 
bles from  the  pores  of  the  wood. 

6.  The  Pneumatic  Balloon.  Fig.  83  repre- 
sents a  small  glass  balloon  with  its  car  im- 
mersed in  a  jar  of  water,  and  placed  under  a 
receiver.  On  exhausting  the  air,  the  air  withm , 
the  balloon  expanding,  gives  it  buoyancy,  and  it 
will  rise  in  the  jar.  On  readmitting  the  air  the 
balloon  will  sink. 

7.  The  experiment  may  be  performed  with- 
out the  air-pump  by  covering  the  jar  with  some 
elastic  substance,  as  india-rubber.  By  pres- 
sing on  the  elastic  covering  with  the  finger 
the^  air  will  be  condensed,  the  water  will  rise 
in  the  balloon,  and  it  will  sink.  On  removing 
the  pressure,  the  air  in  the  balloon  expand- 


Fig.  83 


Explain  the  first  experiment.— 2d.  3d.  4th.   5th.   6th.  7th. 


116 


NATURAL  PHILOSOPHY. 


ing,  will  expel  part  of  the  water  and  the  balloon  will  rise.  This 
is  the  more  convenient  mode  of  performing  the  experiment,  as 
it  can  be  repeated  at  pleasm  e  without  resort  to  the  pump."^ 

8.  The  following  is  a  full  explanation : — The  pressure  on 
the  top  of  the  vessel  first  condenses  the  air  between  the  cover 
and  the  surface  of  the  water; — this  condensation  presses  upon 
the  water  below,  and  as  this  pressure  affects  every  portion 
of  the  water  throughout  its  whole  extent,  the  water,  by  its 
upward  pressure,  compresses  the  air  within  the  balloon,  and 
makes  room  for  the  ascent  of  more  water  into  the  balloon  so 
as  to  alter  the  specific  gravity  of  the  balloon,  and  cause  it  to 
sink.  As  soon  as  the  pressure  ceases,  the  elasticity  of  the  air 
in  the  balloon  drives  out  the  lately  entered  water,  and  restoring 
the  former  lightness  to  the  balloon  causes  it  to  rise.  If,  in  the 
commencement  of  this  experiment,  the  balloon  be  made  to 
have  a  specific  gravity  too  near  that  of  water,  it  will  not  rise 
of  itself,  after  once  reaching  the  bottom,  because  the  pressure 
of  the  water  then  above  it  will  perpetuate  the  condensation  of 
the  air  which  caused  it  to  descend.  It  may  even  then,  how- 
ever, be  made  to  rise,  if  the  perpendicular  height  of  the  water 
above  it  be  diminished  by  inclining  the  vessel  to  one  side. 

9.  This  experiment  proves  many  things  ;  namely  : 

First.  The  materiality  of  air,  hj  X\lq  pressure  of  the  hand 
*  This  experiment  exhibits  the  principle  on  which  the  well-known  glass 
figure,  called  the  Cartesian  Devil,  is  constructed ;  and  it  may  be  thus  ex- 
plained :  several  images  of  glass,  hollow  within,  and  each  having  a  small 
opening  at  the  heel  by  which  water  may  pass  in  and  out,  may  be  made  to 
manoeuvre  in  a  vessel  of  water.  Place  them  in  a  vessel  in  the  same  manner 
with  the  balloon,  but  by  allowing  different  quantities  of  water  to  enter  the 
apertures  in  the  images,  cause  tbem  to  differ  a  little  from  one  another  in 
specific  gravity.  Then,  when  a  pressure  is  exerted  on  the  cover,  the 
heaviest  will  descend  first,  and  the  others  follow  in  the  order  of  their  spe- 
cific gravity ;  and  they  will  stop  or  return  to  the  surface  in  reverse  order, 
when  the  pressure  ceases.  A  person  exhibiting  these  figures  to  spectators 
who  do  not  understand  them,  while  appearing  carelessly  to  rest  his  hand 
on  the  cover  of  the  vessel,  seems  to  have  the  power  of  ordering  their  move- 
ments by  his  will.  If  the  vessel  containing  the  figures  be  inverted,  and 
the  cover  be  placed  over  a  hole  in  the  table,  through  which,  unobserved, 
pressure  can  be  made  by  a  rod  rising  through  the  hole,  and  obeying  the  foot 
of  the  exhibiter,  the  most  surprising  evolutions  may  be  produced  among 
the  figures,  in  perfect  obedience  to  the  word  of  command. 


8.  Explain,  in  full,  the  experiment  of  the  glass  figures.  9.  Explain  all 
that  this  experiment  proves.    Explain  the  condensing  jar. 


PNEUMATICS. 


117 


on  the  top  being  communicated  to  the  water  below  through 
the  ah'  in  the  upper  part  of  the  vessel. 

Secondly.  The  compressibility  of  air,  by  what  happens  m  the 
globe  before  it  descends. 

Thirdly.  The  elasticity,  or  elastic  force  of  air,  when  the 
water  is  expelled  from  the  globe,  on  removing  the  pressure. 

Fourthly.  The  lightness  of  air,  in  the  buoyancy  of  the  globe. 

Fifthly.  It  shows  that  the  pressure  of  a  liquid  is  exerted  m 
all  directions,  because  the  effects  happen  in  whatever  position 
the  jar  be  held. 

Sixthly.  It  shows  that  pressure  is  as  the  depth,  because  less 
pressure  of  the  hand  is  required,  the  farther  the  globe  has  de- 
scended in  the  water.  n  -i 

Seventhly.  It  exemplifies  many  circumstances  of  fluid  sup- 
port. A  person,  therefore,  who  is  familiar  with  this  experi- 
ment, and  can  explain  it,  has  learned  the  principal  truths  ot 
Hydrostatics  and  Pneumatics. 

EXPERIMENTS  W^ITH  CONDENSED  AIR. 

1.  The  Condensing  and  ExHAusTiNa  SvEiNaE.  The  con- 
densing syringe  is  the  air-pump  reversed.^  The  exhausting 
syringe  is  the  simple  air-pump  without  its  plate  or  stand. 
These  implements  are  used  respectively  with  such  parts  ol  the 
apparatus  as  cannot  conveniently  be  attached  to  the  air- 
pump  ;  and  as  an  addition  to 
such  pumps  as  do  not  perform 
the  double  office  of  exhaustion 
and  condensation.  In  some  sets 
of  apparatus  the  condensing  and 
exhausting  syringes  are  united,  and 
are  made  to  perform  each  office 
respectively,  by  merely  reversing 
the  part  which  contains  the  valve. 

2.  The  Air-Chamber.  The 
air-chamber,  Fig.  83,  is  a  hollow 
brass  globe  prepared  for  the  re- 
::,eption  of  a  stop-cock,  and  is  de- 
signed for  the  reception  of  con- 
densed air.  It  is  made  in  different 
forms  in  different  sets,  and  is  used 
by  screwing  it  to  a  condensing 
pump  or  a  condensing  syringe. 


118 


NATURAL  PHILOSOPHY. 


3.  Straight  and  Revolving  Jets  from  Condensed  Air. 
Fill  the  air-chamber  (Fig.  84)  partly  with  water  and  then  con- 
dense the  air.     Then  confine  the  air  by  turning  the  cock; 
after  which  unscrew  it  from  the  air-pump,  and  screw  on  the 
straight  or  the  revolving  jet.    Then  open  the  stop-cock,  and  the 
water  will  be  thrown  from  the  chamber  in  the  one  case,  in 
a  straight  continued  stream,  in  the  other  in  the  form  of  a 
wheel.    Figs.  85  and  86 
represent  a  view  of  the 
straight  and  the  revolving 
jets.   In  the  revolving  jet 
the  water  is  thrown  from 
two  small  apertures  made 

at  each  end  on  opposite  sides,  to  assist  the  revolution. 
The  circular  motion  is  caused  by  the  reaction  of  the 
water  on  the  sides  of  the  arms  opposite  the  jets ;  for 
as  the  water  is  forced  into  the  tubes,  it  exerts  an  equal 
pressure  on  all  sides  of  the  tubes,  and  as  the  pressure  is  re- 
lieved on  one  side  by  the  jet-hole,  the  arm  is  caused  to  revolve 
in  a  contrary  direction.  This  experiment  performed  with  the 
straight  jet,  illustrates  the  principle  on  which  "  Hero's  ball" 
and  Hero's  fountain  are  constructed. 

4.  The  Principle  of  the  Air-gun.  With  the  air-chamber 
as  in  the  last  experiments,  a  small  brass  cyHnder  or  gun-barrel, 
Fig.  87,  may  be  substituted  for  the  jets,"  and  loaded 

with  a  small  shot  or  paper  ball.    On  turning  the     rig.  87. 
cock  quickly,  -the  condensed  air  rushing  out  will  throw  _ 
the  shot  to  a  considerable  distance.    In  this  way  the 
air-gun  operates,  an  apparatus  resembling  the  lock  of 
a  gun  being  substituted  for  the  stop-cock,  by  which  a        1 1 
small  portion  only  of  the  condensed  air  is  admitted  to       |  j  l 
escape  at  a  time,  so  that  the  chamber  being  once  filled  • 
will  afford  two  or  three  dozen  discharges.    The  force  W 
of  the  air-gun  has  never  been  equal  to  more  than  a 
fifteenth  of  the  force  of  a  common  charge  of  powder,  and  the 
loudness  of  the  report  made  in  its  discharge  is  always  as  great 
in  proportion  to  its  force  as  that  of  the  common  gun. 

5.  Condensed  air  may  be  weighed  in  the  air-chamber;  but 
m  estimating  its  weight,  the  temperature  of  the  room  must  al- 
ways be  taken  into  consideration,  as  the  density  of  air  is 
materially  affected  by  heat  and  cold. 


Explain  the  jets.    The  air-gun 


PNEUMATICS. 


119 


EXPERIMENTS  SHOWING  THE  INERTIA  OF  AIR. 

The  Guinea  and  Feather  Drop.    The  inertia  of  air  is 
shown  by  the  guinea  and  feather  drop,  exhibiting  the  resistance 
which  the  air  opposes  to  falhng  bodies.  This  apparatus  is  made 
in  different  forms,  some  having  shelves      j,.^  gg 
Fig.  88.    on  w^hich  the  guinea  and  feather  rest ;  ° 
and  when  the  air  is  exhausted  they 
are  made  to  fall  by  the  turning  of 
a  handle.    A  better  form  is  that  rep- 
resented in  Fig.   88,  in  which  the 
guinea  and  feather  (or  a  piece  of 
brass  substituted  for  the  guinea)  are 
enclosed,   and  the   apparatus  being 
\     \     screwed  to  the  plate  of  the  pump, 
the  air  is   exhausted ;    a  stop-cock 
^       turned  to  prevent  the  readmission  of 
the  air,  and  the  apparatus  being  then 
unscrewed,  the  experiment  may  be  repeatedly 
showed  by  one  exhaustion  of  the  air.    It  will 
then  appear  that  every  time  the  apparatus  is 
inverted,  the  guinea  and  the  feather  will  fall 
simultaneously.    The  two  forms  of  the  guinea 
and  feather  drop  are  exhibited  in  Figs.  88  and 
89,  one  of  which.  Fig.  88,  is  furnished  with  a  stop-cock,^  the 
other,  Fig.  89,  with  shelves. 


EXPERIMENTS  SHOWING  THE  FLUIDITY  OF  AIR. 

1.  The  Weight-lifter.  The  upward  pressure  of  the  air, 
one  of  the  properties  of  its  fluidity,  may  be  exhibited  by  an 
apparatus  called  the  weight-lifter,  made  in  different  forms, 
but  all  on  the  same  principle.    The  one  represented  in  Fig.  90 

*  Most  sets  of  philosophical  apparatus  are  furnished  with  stop-cocks 
and  elastic  tubes,  for  the  purpose- of  connecting  the  several  parts  with  the 
pump  or  with  one  another.  In  selecting  the  apparatus,  it  is  important 
to  have  the  screws  of  the  stop-cocks,  and  of  all  the  apparatus  of  similar 
thread,  in  order  that  every  article  may  subserve  as  many  purposes  as 
possible.  This  precaution  is  suggested  by  economy  as  well  as  by  conve- 
nience. 


Explain  the  experiment  with  the  guinea  and  feather.  Also  the  weight- 
lifter.  . 


120 


NATURAL  PHILOSOPHY. 


consists  of  a  glass  tube,  of  large 
bore,  set  in  a  strong  case  or  stand.  Fig.  90. 

supported  by  three  legs.  A  piston  is 
accurately  fitted  to  the  bore  of  the 
tube,  and  a  hook  is  attached  to  the  bot- 
tom of  the  piston,  from  which  weights 
are  to  be  suspended.  One  end  of  the 
elastic  tube  is  to  be  screwed  to  the 
plate  of  the  pump,  and  the  other  end 
attached  to  the  top  of  this  instrument. 
The  air  being  then  exhausted  from  the 
tube,  the  weights  will  be  raised  the 
whole  length  of  the  glass.  The  num- 
ber of  pounds  weight  that  can  be  raised 
by  this  instrument  may  be  estimated  by  multiplying  the  num-- 
ber  of  square  inches  in  the  bottom  of  the  piston  by  fifteen. 

2.  The  Pneumatic  Shower-bath.  On  the  principle  of  the 
upward  pressure  of  the  air,  the  pneumatic  shower-bath  is 
constructed.  It  consists  of  a  tin  vessel  perforated  with  holes 
in  the  bottom  for  the  shower,  and  having  an  aperture  at  the 
top,  which  is  opened  or  closed  at  pleasure  by  means  of  a  spring 
valve.  [Instead  of  the  spring  valve,  a  bent  tube  may  be 
brought  round  from  the  top  down  the  side  of  the  vessel,  with 
an  aperture  in  the  tube  below  the  bottom  of  the  vessel  which 
may  be  covered  with  the  thumb.]  On  immersing  the  vessel 
thus  constructed  in  a  pail  of  water,  with  the  valve  open,  and 
the  tube  (if  it  have  one)  on  the  outside  of  the  pail,  the  water 
will  fill  the  vessel.  The  aperture  then  being  closed  with  the 
spring  or  with  the  thumb,  and  the  vessel  being  hfted  out  of 
the  water,  the  upward  pressure  of  the  air  will  confine  the 
water  in  the  vessel.  On  removing  the  thumb,  or  opening  the 
valve,  the  water  will  descend  in  a  shower  until  the  vessel  is 
emptied. 

MISCELLANEOUS   EXPERIMENTS   DEPENDING   ON   TWO  OR 
MORE  OF  THE  PROPERTIES  OF  AIR. 

1.  The  Bolt-head  and  Jar.  Fig.  91,  a  glass  globe  with 
a  long  neck,  called  a  bolt-head,  (or  any  long-necked  bottle,) 
partly  filled  with  water,  is  inverted  in  a  jar  of  water,  (colored 
with  a  few  drops  of  red  ink  or  any  coloring  matter,  in  order  that 


Explain  the  pneumatic  shower-bath. 


PNEUMATICS. 

the  effects  may  be  more  distinctly  visible,)  and 
placed  under  the  receiver.  On  exhausting  the  air 
in  the  receiver,  the  air  in  the  upper  part  of  the  bolt- 
head  expanding,  expels  the  water,  showing  the 
elasticity  of  the  air.  On  readmitting  the  air  to 
the  receiver,  as  it  cannot  return  into  the  bolt- 
head,  the  pressure  on  the  surface  of  the  water  in 
the  jar,  forces  the  water  into  the  bolt-head, 
showing  the  pressure  of  the  air  caused  by  its 
weight.  The  experiment  may  be  repeated  with 
the  bolt-head  without  any  water,  and  on  the  re- 
admission  of  the  air,  the  water  will  nearly  fill 
the  bolt-head,  affording  an  accurate  test  of  the 
degree  of  exhaustion. 

2.  The  Transfer  of  Fluids  from  one  Vessel  to  another. 
The  experiment  may  be  made  with  two  bottles  tightly  closed. 
Let  one  be  partly  filled  with  water,  and  the  two  connected  by  a 
bent  tube,  connecting  the  interior  of  the  empty  bottle  with  the 
water  of  the  other,  and  extending  nearly  to  the  bottom  of  the 
water.  On,,  exhausting  the  air  from  the  empty  bottle,  the 
water  will  pass  to  the  other,  and  on  readmitting  the  air  the 
water  will  return  to  its  original  position,  so  long  as  the  lov/er 
end  of  the  bent  tube  is  above  the  surface. 

EXPERIMENTS  WITH  THE  SIPHON. 

1.  Close  the  shorter  end  of  the  siphon  with  the  finger  or 
with  a  stop-cock,  and  pour  mercury  or  water  into  the  longer 
side.  The  air  contained  in  the  shorter  side  will  prevent  the 
liquid  from  rising  in  the  shorter  side."  But  if  the  shorter  end 
be  opened,  so  as  to  afford  free  passage  outwards  for  the  air, 
the  fluid  will  rise  to  an  equilibrium  in  both  arms  of  the  si- 
phon. 

2.  Pour  any  liquid  into  the  longer  arm  of  the  siphon  until 
the  shorter  arm  is  filled.  Then  close  the  shorter  end,  to  pre- 
vent the  admission  of  the  air ;  the  siphon  may  then  be  turned 
in  any  direction  and  the  fluid  will  not  run  out,  on  account  of 
the  pressure  of  the  atmosphere  against  it.  But  if  the  shorter 
end  be  unstopped,  the  fluid  will  run  out  freely. 


Explain"  the  transfer  of  fluids.  Explain  the  first  experiment  with  the 
siphon.    Also  the  second. 

6 


122 


NATURAL  PHILOSOPHY. 


AIR  ESSENTIAL  TO  ANIMAL  LIFE. 

If  an  animal  be  placed  under  the  I'eceiver,  and  the  air  ex- 
hausted, it  will  immediately  droop,  and  if  the  air  be  not 
speedily  readmitted  it  will  die. 

AIR  ESSENTIAL  TO  COMBUSTION. 

Place  a  lighted  taper,  cigar,  or  any  other  substance  that 
will  produce  smoke,  under  the  receiver,  and  exhaust  the  air ; 
the  light  will  be  extinguished,  and  the  smoke  will  fall,  instead 
of  rising.    If  the  air  be  readmitted,  the  smoke  will  ascend. 

THE  PRESSURE  OF  THE  AIR  RETARDS  EBULLITION.* 

1.  Ether,  alcohol,  and  other  distilled  liquors,  or  boiling 
water,  placed  under  the  receiver,  will  appear  to  boil  when 
the  air  is  exhausted. 

2.  The  existence  of  many  bodies  in  a  liquid  form  depends 
on  the  weight  or  pressure  of  the  atmosphere  upon  them.  The 
same  force,  likewise,  prevents  the  gases  which  exist  in  fluid 
and  solid  bodies  from  disengaging  themselves.  If,  by  i-ai  efy- 
ing  the  air,  the  pressure  on  these  bodies  be  diminished,  they 
either  assume  the  form  of  vapors,  or  else  the  gas  detaches 
itself  altogether  from  the  other  body.  The  following  experi- 
ment proves  this :  place  a  quantity  of  lukewarm  water,  milk, 
or^alcohol,  under  a  receiver,  and  exhaust  the  air,  and  the  liquid 
will  either  pass  off  in  vapor,  or  will  have  the  appearance  of 
boilmg. 

3.  An  experiment  to  prove  that  the  pressure  of  the  atmo- 
sphere preserves  some  bodies  in  the  liquid  form,  may  thus  be 
performed :  fill  a  long  ^^al,  or  a  tube  closed  at  one  end,  with 
water,  and  invert  it  in  a  vessel  of  water.  The  atmospheric 
pressure  will  retain  the  water  in  the  vial.  Then  by  means 
of  a  bent  tube  introduce  a  few  drops  of  sulphuric  ether,  which, 
by  reason  of  their  small  specific  gravity,  will  ascend  to  the  top 

*  Ebullition.  The  operation  of  boiling.  The  agitation  of  liquor  by 
heat,  which  throws  it  up  into  bubbles. 


What  takes  place  when  an  animal  is  placed  under  an  exhausted  re- 
ceiver?   Is  air  essential  to  combustion  ?    How  is  this  proved? 

How  is  it  shown  that  air  prevents  ebullition  ?  Give  the  1st,  2d,  and  3d 
experiments. 


TNEUMATICS. 


123 


of  the  vial,  expelling  an  equal  bulk  of  water.  Place  the  whole 
under  the  receiver,  and  exhaust  the  air,  and  the  ether  will  be 
seen  to  assume  the  gaseous  form,  expanding  in  proportion  to 
the  rarefaction  of  the  air  under  the  receiver,  so  that  it  gradually 
expels  the  water  from  the  vial,  and  fills  up  the  entire  space 
itself.  On  readmitting  the  air,  the  ether  becomes  condensed, 
and  the  water  will  reascend  into  the  vial. 

4.  A  simple  and  interesting  experiment  connected  with  the 
science  of  chemistry,  may  thus  be  performed  by  means  of  the 
air-pump.  A  watch-glass,  containing  water,  is  placed  over  a 
small  vessel  containing  sulphuric  acid,  and  put  under  the 
bulbed  receiver.  When  the  air  is  exhausted,  vapor  will  freely 
rise  from  the  water,  and  be  quickly  absorbed  by  the  acid.  An 
intense  degree  of  cold  is  thus  produced,  and  the  water  will 
freeze. 

5.  In  the  above  experiment,  if  ether  be  used  instead  of  the 
acid,  the  ether  will  evaporate  instead  of  the  water,  and  in  the 
process  of  evaporation,  depriving  the  water  of  its  heat,  the 
water  will  freeze.  These  two  experiments,  apparently  similar 
in  effects,  namely,  the  freezing  of  the  water,  depend  upon  two 
different  principles  which  pertain  to  the  science  of  chemistry. 

THE  PNEUMATIC  PARADOX. 

An  interesting  experiment,  illustrative  of  the  pneumatic 
paradox,  may  be  thus  performed : — Pass  a  small  open  tube, 
(as  a  piece  of  quill,)  through  the  centre  of  a  circular  card  two 
or  three  inches  in  diameter,  and  cement  it,  the  lower  end  pass- 
ing down,  and  the  upper  just  even  with  the  card.  Then  pass 
a  pin  through  the  centre  of  another  similar  card,  and  place  it 
on  the  former  with  the  pin  projecting  into  the  tube  to  prevent 
the  upper  card  from  shding  off.  It  will  then  be  impossible  to 
displace  the  upper  card  by  blowing  through  the  quill,  on  ac- 
count of  the  adhesion  produced  by  the  current  passing  between 
the  discs.  On  this  principle,  smoky  chimneys  have  been  rem- 
edied, and  the  office  of  ventilation  more  effectually  performed. 


Give  the  4th  experiment  to  show  that  air  prevents  ebulhtion.  Also  the 
fifth.    Explain  the  pneumatic  paradox. 


121 


NATURAL  PHILOSOPHY. 


CHAPTER  VIII. 

ACOUSTICS, 

152.  Acoustics  is  the  science  which  treats  of  the 
nature  and  laws  of  sound.  It  includes  the  theory  of 
musical  concord  or  harmony. 

153.  Sound  is  caused  by  a  tremulous  or  vibratory 
motion  of  the  air. 

1.  If  a  bell  be  rung  under  an  exhausted  receiver,  no  sound 
can  be  heard  from  it ;  but  when  the  air  is  admitted  to  surround 
the  bell,  the  vibrations  immediately  produce  sound. 

2.  Again,  if  the  experiment  be  made  by  enclosing  the  bell  in 
a  small  receiver,  full  of  air,  and  placing  that  under  another 
receiver,  from  which  the  air  can  be  withdrawn ;  though  the  bell, 
when  struck,  must  then  produce  sound,  as  usual,  yet  it  will 
not  be  heard  if  the  outer  receiver  be  well  exhausted,  and  care 
be  taken  to  prevent  the  vibrations  from  being  communicated 
through  any  solid  part  of  the  apparatus ;  because  there  is  no 
medium  through  which  the  vibrations  of  the  bell,  in  the  smaller 
receiver,  can  be  communicated  to  the  ear. 

154.  Sounds  are  louder  when  the  air  surrounding  the 
sonorous  body  is  dense,  than  when  it  is  in  a  rarefied 
state. 

For  this  reason  the  sound  of  a  bell  is  louder  in  cold  than  in 
warm  weather;  and  sound  of  any  kind  is  transmitted  to  a 
greater  distance  in  cold,  clear  weather,  than  in  a  warm  sultry 
day.  On  the  tops  of  mountains,  where  the  air  is  rare,  the  hu- 
man voice  can  be  heard  only  at  the  distance  of  a  few  rods  ; 
and  the  firing  of  a  gun  produces  a  sound  scarcely  louder  than 
the  cracking  of  a  whip. 


152.  What  is  that  science  called  which  treats  of  the  nature  and  laws  of 
sound?    What  does  it  include? 

153.  What  causes  sound?    What  illustrations  are  given  to  prove  this? 

154.  In  what  proportion  are  sounds  loud  or  faint  ?  Why  does  a  bell 
sound  louder  in  cold  than  in  warm  weather?  Why  is  sound  fainter  on  the 
top  of  a  mountain  than  near  the  surface  of  the  earth  ? 


ACOUSTICS. 


125 


15r>.  Sonorous  bodies  are  those  which  produce  clear, 
distinct,  regular,  and  durable  sounds,  ^uch  as  a  bell,  a 
drum,  wind  instruments,  musical  strings  and  glasses. 
These  vibrations  can  be  communicated  to  a  distance 
not  only  through  the  air,  but  also  through  liquids  and 
solid  bodies. 

156.  Bodies  owe  their  sonorous  property  to  their 
elasticity.* 

157.  The  sound  produced  by  a  musical  string  is 
caused  by  its  vibrations  ;  and  the  height  or  depth  of  the 
tone  depends  upon  the  rapidity  of  these  vibrations. 
Long  strings  vibrate  with  less  rapidity  than  short  ones, 
and  for  this  reason  the  low  tones  in  a  musical  instru- 
ment proceed  from  the  long  strings,  and  the  high  tones 
from  the  short  ones. 

1.  Fig.  92,  AB  represents  a  musical  string.    If  it  be  drawn 

up  to  G,  its  eks-  ^. 

,  Fig.  9a 

ticity  will  not  on-  ^ 

ly  carry  it  back 

again,  but  will  "''.V'-'^-C  iTr'^'^'^^'V*^^., 

give  it  a  momen-  ^   

turn  which  will   -d  Z^-'^^^^^^ 

carry  it  to  H,  """^--I'^^^  F  "^l"""'^' 

from  whence  it   -^j  ""' 

will  successively 

return  to  T,  F,  C,  D,  &c.,  until  the  resistance  of  the  air  entirely 
destroys  its  motion. 

2.  the  vibrations  of  a  sonorous  body  give  a  tremulous  mo- 
tion to  the  air  around  it,  similar  to  the  motion  communicated 
tC'  smooth  water  when  a  stone  is  thrown  into  it. 

*  Although  it  is  undoubtedly  the  case  that  all  sonorous  bodies  are  elas- 
tic, it  is  not  to  be  inferred  that  all  elastic  bodies  are  sonorous. 


155.  V^hat  are  sonorous  bodies? 

156.  To  what  do  sonorous  bodies  owe  their  sonorous  property?  Are  all 
elastic  bodies  sonorous  ? 

157.  What  causes  the  sound  produced  by  a  musical  string?  Upon  what 
does  the  height  and  depth  of  the  tone  depend  ?  Which  strings,  in  a  mu^ 
Bical  instrument,  produce  the  low  tones?    Why?    Explain  Fig.  92. 


126 


NATURAL  PHILOSOPHY. 


158.  The  science  of  harmony  is  founded  on  the  rela- 
tion which  the  vibrations  of  sonorous  bodies  have  to 
each  other. 

Thus,  when  the  vibrations  of  one  string  are  double  those  of 
another,  the  chord  of  an  octave  is  produced.  If  the  vibrations 
of  two  strings  be  as  two  to  three,  the  chord  of  a  fifth  is  pro- 
duced.^ When  the  vibrations  of  two  strings  frequently  coin- 
cide, they  produce  a  musical  chord  ;  and  when  the  coincidence 
of  the  vibrations  is  unfrequent,  discord  is  produced. 

159.  The  quality  of  the  sound  produced  by  strings 
depends  upon  their  length,  thickness,  weight,  and  degree 
of  tension.  The  quality  of  the  sound  produced  by  wind 
instruments  depends  upon  their  size,  their  length,  and 
their  internal  diameter. 

Long  and  large  strings,  when  loose,  produce  the  lowest 
tones  ;^but  different  tones  maybe  produced  from  the  same 
string,  according  to  the  degree  of  tension.  Large  wind  in- 
struments, also,  produce  the  lowest  tones ;  but  different  tones 
may  be  produced  from  the  same  instrument,  according  to  the 
distance  of  the  aperture  for  the  escape  of  the  wind,  from  the 
aperture  where  it  enters. 

160.  The  quality  of  the  sound  of  all  musical  instru- 
ments is  affected  by  the  changes  in  the  temperature  and 
specific  gravity  of  the  atmosphere. 

As  heat  expands  and  cold  contracts  the  materials  of  which 

*  When  music  is  made  by  the  use  of  strings,  the  air  is  struck  by  the 
body,  and  the  sound  is  caused  by  the  vibrations ;  when  it  is  made  by 
pipes,  the  body  is  struck  by  the  air  ;  but  as  action  and  reaction  are  equal, 
the  effect  is  the  same  in  both  cases. 


158.  Upon  what  is  the  science  of  harmony  founded  ?  How  is  the  chord 
of  an  octave  produced?  How  is  the  chord  of  a  fifth  produced?  How  is  a 
musical  chord  produced  ?    A  discord  ? 

159.  Upon  what  does  the  quality  of  the  sound  produced  by  strings  de- 
pend? Upon  what  does  that  produced  by  wind  instruments  depend? 
What  strings  produce  the  lowest  tones  I  How  may  different  tones  be  pro- 
duced from  the  same  string  ?  How  may  different  tones  be  produced  from 
the  same  wind  instrument  ? 

160.  What,  in  som.e  degree,  affects  the  quality  of  the  sound  of  all  musi- 
cal instruments?  What  effect  have  heat  and  cold  on  the  materials  of 
which  the  instrument  Is  made  ?    What  follows  from  this  ? 


ACOUSTICS. 


127 


the  instrument  is  made,  it  follows,  that  the  strings  will  have  a 
greater  degree  of  tension,  and  that  pipes  and  other  wind  in- 
struments Avill  be  contracted,  or  shortened,  in  cold  weather. 
For  this  reason,  most  musical  instruments  are  higher  m  tone 
(or  sharper)  in  cold  weather,  and  lower  in  tone  (or  more  fiat) 
ill  warm  weather. 

IGl.  Sound  is  communicated  more  rapidly  and  with 
greater  power  through  solid  bodies,  than  through  the 
air,  or  fluids.  It  is  conducted  by  water  about  four 
times  quicker  than  by  air,  and  by  solids  about  twice  as 
rapidly  as  by  water. 

1.  If  a  person  lay  his  head  on  a  long  piece  of  tinaber,  he 
can  hear  the  scratch  of  a  pin  at  the  other  end,  while  it  could 
not  be  heard  through  the  air. 

2.  If  the  ear  be  placed  against  a  long,  dry,  brick  wall,  and 
a  person  strike  it  once  with  a  hammer,  the  sound  will  be  heard 
twice,  because  the  wall  will  convey  it  with  greater  rapidity 
than  the  air,  though  each  will  bring  it  to  the  ear. 

162.  The  Stethescope  is  an  instrument  depending  on 
the  power  of  solid  bodies  to  convey  sound. 

It  consists  of  a  wooden  cylinder,  one  end  of  which  is  applied 
firmly  to  the  breast,  while  the  other  end  is  brought  to  the  ear. 
By  this  means  the  action  of  the  lungs  may  be  distinctly  heard. 
The  instrument,  therefore,  becomes  useful  in  the  hands  of  a 
skilful  physician,  to  ascertain  the  state  of  those  organs. 

163.  Sound,  passing  through  the  air,  moves  at  the 
rate  of  1142  feet  in  a  second  of  time.  This  is  the  case 
with  all  kinds  of  sound. 

1.  The  softest  whisper  flies  as  fast  as  the  loudest  thunder, 

Why  are  most  musical  instruments  higher  in  tone,  or  sharper,  in  cold 
weather  ? 

161.  Through  which  is  sound  communicated  more  rapidly,  and  with 
irreater  power,  through  solid  bodies,  or  the  air?  How  fast  is  it  conducted 
by  water?  How  fast  by  solids?  What  examples  are  given  to  show  that 
sound  is  communicated  more  rapidly  through  solid  bodies  than  the  air  or 
fluids? 

162.  What  is  a  stethescope?  Of  what  does  it  consist?  For  what  is  it 
used  ? 

163.  How  fast  does  sound  move?  Does  the  force  or  direction  of  the 
v</iiid  make  any  difference  in  its  velocity  ? 


128 


NATURAL  PHILOSOPHY. 


and  the  force  or  direction  of  the  wind  makes  but  shght  dif- 
ference in  its  velocity. 

2.  This  uniform  velocity  of  sound  enables  us  to  determine 
the  distance  of  an  object  from  which  it  proceeds.  If,  for  in- 
stance, the  light  of  a  gun,  fired  at  sea,  be  seen  a  half  of  a 
minute  before  the  report  is  heard,  the  vessel  must  be  at  the 
distance  of  six  miles  and  a  half.  In  the  same  manner,  the  dis- 
tance of  a  thunder-cloud  may  be  ascertained,  by  counting  the 
seconds  between  the  appearance  of  the  lightning  and  the  noise 
of  the  thunder,  and  muUiplying  them  by  1142  feet. 

164.  An  echo  is  produced  by  the  vibrations  of  the  air 
meeting  a  hard  and  regular  surface,  such  as  a  wall,  a 
rock,  a  mountain,  and  being  reflected  back  to  the  ear, 
thus  producing  the  same  sound  a  second,  and  sometimes 
a  third  and  fourth  time.* 

1.  Speaking-trumpets  are  constructed  on  the  principle  of 
the  reflection  of  sound. 

2.  The  voice,  instead  of  being  difl'used  in  the  open  air,  is 

*  From  this  it  is  evident  that  no  echo  can  be  heard  at  sea,  or  on  an  ex- 
tensive plain  ;  because  there  is  no  object  there  to  reflect  the  sound.  An 
echo  is  heard  only  when  a  person  stands  in  such  a  situation  as  to  hear 
both  the  original  and  the  reflected  sound.  The  pupil  will  doubtless  recol- 
lect what  has  been  said  in  Mechanics  with  respect  to  the  angles  of  inci- 
dence and  reflection.  Sound  (as  well  as  light,  as  will  be  explained  under 
the  head  of  Optics)  is  communicated  and  reflected  by  the  same  law, 
namely,  that  the  angles  of  incidence  and  reflection  are  always  equal.  It 
is  not  difficult,  therefore,  to  ascertain  the  direction  in  which  sound  will  pro- 
ceed, whether  it  be  direct  or  reflected.  It  is  related  of  Dionysius,  the 
tyrant  of  Sicily,  that  he  had  a  dungeon,  (called  the  ear  of  Dionysius,)  in 
which  the  roof  was  so  constructed  as  to  collect  the  words,  and  even  the 
whispers,  of  the  prisonei-s  confined  therein,  and  direct  them  along  a  hidden 
conductor  to  the  place  where  he  sat  to  listen:  and  thus  he  became  ac- 
quainted with  the  most  secret  expressions  of  his  unhappy  victims. 


What  advantage  results  from  this  uniform  velocity  of  sound  ?  How  can 
the  distance  of  a  thunder-cloud  be  ascertained  ? 

164.  How  is  an  echo  produced  ?  Note.  Why  cannot  an  echo  be  heard 
at  sea,  or  on  an  extensive  plain  ?  How  must  a  person  stand  in  order  to 
hear  an  echo?  By  what  law  is  sound  communicated  and  reflected?  What 
anecdote  is  related  of  Dionysius?  Upon  what  principle  are  speaking- 
trumpets  constructed?  Explain  the  manner  in  which  the  vibrations  of  the 
air  are  reflected. 


A(X)USTICS. 


129 


confined  within  the  trumpet;  and  the  vibrations  which  spread 
and  fall  against  the  sides  of  the  instrument  are  reflected  ac- 
cording to  the  angle  of  incidence,  and  fall  in  the  direction  of 
the  vibrations,  which  proceed  straight  forward.  The  whole 
of  the  vibrations  are  thus  collected  into  a  focus  ;  and  if  the  ear 
be  situated  in  or  near  that  spot,  the  sound  will  be  prodigiously 
increased. 

3.  Hearing-trumpets,  or  the  trumpets  used  by  deaf  persons, 
are  also  constructed  on  the  same  principle ;  but  as  the  voice 
enters  the  large  end  of  the  trumpet,  instead  of  the  small  one, 
it  is  not  so  much  confined,  nor  so  much  increased. 

4.  The  musical  instrument  called  the  trumpet,  acts,  also,  on 
the  same  principle  with  the  speaking-trumpet,  so  far  as  its 
form  tends  to  increase  the  sound. ^ 

165.  Sound,  like  light,  after  it  has  been  reflected 
from  several  surfaces,  may  be  collected  into  one  point, 
as  a  focus,  where  it  will  be  more  audible  than  in  any 
other  part  ;  and  on  this  principle  whispering-galleries 
may  be  constructed. 

The  famous  whispering-gallery  in  the  dome  of  St.  Paul's 
church,  in  London,  is  constructed  on  this  principle.f  Persons 
at  very  remote  parts  of  the  building  can  carry  on  a  conversa- 
tion in  a  soft  whisper,  which  will  be  distinctly  audible  to  one 
another,  while  others  in  the  building  cannot  hear  it ;  and  the 
ticking  of  a  watch  may  be  heard  from  side  to  side. 

*  The  smooth  and  polished  surface  of  the  interior  parts  of  certain  kinds 
of  shells,  particularly  if  they  be  spiral  or  undulating,  fit  them  to  collect  and 
reflect  the  various  sounds  which  are  taking  place  in  the  vicinity.  Hence 
the  Cyprias,  the  Nautilus,  and  some  other  shells,  when  held  near  the  ear, 
give  a  continued  sound,  which  resembles  the  roar  of  the  distant  ocean. 

t  There  is  a  church  in  the  town  of  Newburyport,  in  Massachusetts, 
which,  as  was  accidentally  discovered,  has  the  same  property  as  a  whis- 
penng-gallery.  Persons  in  opposite  corners  of  the  building,  by  facing  the 
wall,  may  carry  on  a  conversation  in  the  softest  whisper,  unnoticed  by 
others  in  any  other  part  of  the  building.  It  is  the  building  which  contains 
in  its  cemetery  the  remains  of  the  distinguished  preacher,  Whitfield. 


Upon  what  principle  are  hearing-trumpets  constructed  ?  How  far  does 
the  musical  instrument,  called  the  trumpet,  act  upon  the  principle  of  the 
speaking-trumpet?  Note.  How  can  the  continued  sound,  given  by  some 
shells,  when  held  near  the  ear,  be  explained  ? 

165.  Upon  what  principle  may  whispering-galleries  be  constructed? 


130 


NATURAL  PHILOSOPHY. 


166.  Sounds  may  be  conveyed  to  a  much  greater 
distance  through  contmuous  tubes  than  through  the 
open  air. 

1.  The  tubes  used  to  convey  sounds  are  called  acoustic 
tubes.  They  are  much  used  in  pubhc  houses,  stores,  count- 
ing-rooms, &c.,  to  convey  communications  from  one  room  to 
another. 

2.  The  quality  of  sound  is  affected  by  the  furniture  of  a 
room,  particularly  the  softer  kinds,  such  as  curtains,  carpets, 
(fee,  because,  ha^ang  Httle  elasticity,  they  present  surfaces  un- 
favorable to  vibrations. 

3.  For  this  reason,  music  always  sounds  better  in  rooms  with 
bare  walls,  without  carpets,  and  without  curtains.  For  the 
same  reason,  a  crowded  audience  increases  the  difficulty  of 
speaking. 

4.  As  a  general  rule,  it  may  be  stated,  that  plane  and 
smooth  surfaces  reflect  sound  without  dispersing  it ;  convex  sur- 
faces dis2oerse  it,  and  concave  surfaces  collect  it. 

5.  The  air  is  a  better  conductor  of  sound  when  it  is  humid 
than  when  it  is  dry. 

6.  A  bell  can  be  more  distinctly  heard  just  before  a  rain  ; 
and  sound  is  heard  better  in  the  night  than  in  the  day,  because 
the  air  is  generall}^  more  damp  in  the  night. 

7.  The  distance'to  which  sound  may  be  heard  depends  upon 
various  circumstances,  on  which  no  definite  calculations  can  be 
predicated.  Volcanoes,  among  the  Andes,  in  South  America, 
have  been  heard  at  the  distance  of  three  hundred  miles  ;  naval 
engagements  have  been  heard  two  hundred ;  and  even  the 
watchword  ''All's  ivell,''  pronounced  by  the  unassisted  human 
voice,  has  been  heard  from  Old  to  i^ew  Gibraltar,  a  distance  of 
twelve  miles. 

167.  The  sound  of  the  human  voice  is  produced  by 
the  vibration  of  two  dehcate  membranes,  situated  at  the 
top  of  the  windpipe,  and  between  which  the  air  from  the 
lungs  passes. 

166.  In  what  way  can  sounds  be  conveyed  to  a  much  greater  distance 
than  through  the  air?  What  are  the  tubes,  used  to  convey  sounds,  called  ? 
W^hy  do  the  softer  kinds  of  furniture  in  a  room  affect  the  quality  of  the 
sound  ?  What  general  rule  is  given  with  regard  to  the  reflection  of  sound  ? 
Is  thoiiair  .a  better  conductor  when  it  is  humid,  or  when  it  is  dry  ?  Why 
can  a  sound  be  heard  better  in  the  night  than  in  the  day? 

167.  How  is  the  sound  of  the  human  voice  produced? 


ACOUSTICS. 


131 


1.  The  tones  are  varied  from  grave  to  acute,  by  opening  or 
contracting  the  passage ;  and  they  are  r(.*gulated  by  the  naus- 
cles  bcilonging  to  the  throat,  by  the  tongue,  and  by  the  cheeks. 

2.  The  management  of  the  voice  depends  much  upon  culti- 
vali(jn  ;  and  although  many  persons  can  both  speak  and  sing 
willi  ease,  and  with  great  power,  without  much  attention  to  its 
culture,  yet  it  is  found  that  they  who  cultivate  their  voices  by 
use,  acquire  a  degree  of  liexibility  and  ens(;  in  its  management, 
which,  in  a  great  measure,  supplies  tli^;  (\<-\\<:\^-ni:y  (>[  nature.* 

3.  Yentriloquismf  is  the  art  of  speaking  in  such  a  manner 
as  to  cause  the  voice  to  appear  to  proceed  from  a  distance. 

*  The  reader  is  referred  to  Dr.  Rush's  very  valuable  work  oii  the 
Philosophy  of  the  Huruaii  Voice,"  for  plain  and  jjraetieiii  instructions  on 
this  Buhject.  Dr.  Barber's  "  Grammar  of  Eloculioji,  '  and  Purkor's  "  Pro- 
gressive Exercises  iji  Rlielorical  j{<'-adinrf,"  likewise  contain  the  same  in- 
structions in  a  practical  form.  To  thfj  work  of  Dr.  Rush,  both  of  the 
latter-mentioned  works  arc  jjjrcr.  iy  ind-'-bl/  d. 

t  The  word  ventj-iloqiiism  lit-'-r-; I i;  ijir  ^ms,  speakinrr  from  the  helhj,'' 
and  it  is  so  defined  in  Chamljt-rs'  Dictionary  of  Arts  and  Sciences.  The 
ventriiofjuist,  by  a  sinj^uiar  Kvnio/nuuf'nt  of  tfie  voice,  seems  to  have  it  ia 
his  power  "  to  throuj  his  voice''  in  any  direction,  so  that  the  sound  shall  ap- 
pear to  proceed  from  that  spot.  The  words  are  pronounced  by  the  or^rans 
usually  employed  for  that  purpose,  but  in  such  a  manner  as  to  f^ive  httle 
or  no  motion  to  the  lips,  the  organs  chiefly  concerned  bein^  tho.:e  of  the 
throat  and  tongue.  The  variety  of  sounds  which  the  human  voice  is  ca- 
pable of  thus  producing  is  altogether  beyond  common  belief,  and,  indeed, 
is  truly  surprising.  Adepts  in  this  art  will  mimic  the  voices  of  all  ages 
and  conditions  of  human  life,  from  the  smallest  infant  to  the  tremulous 
voice  of  tottering  age,  and  from  the  intoxicated  foreign  beggar  to  the 
high-bred,  artificial  tones  of  the  fashionable  lady.  Some  will  also  imitate 
the  warbling  of  the  nightingale,  the  loud  tones  of  the  whip-poor-will,  and 
the  scream  of  the  peacock,  with  equal  truth  and  facility.  Nor  are  these 
arts  confined  to  professed  imitators  ;  for  in  many  villages  boys  may  be 
found,  who  are  in  the  habit  of  imitating  the  brawling  and  spitting  of  cats, 
in  such  a  manner  as  to  deceive  almost  every  hearer. 

The  human  voice  is  also  capable  of  imitating  almost  every  inanimate 
sound.  Thus,  the  turning  and  occasional  creaking  of  a  grindstone,  with 
the  rush  of  the  water,  the  sawing  of  wood,  the  trundling  and  creaking  of  a 
wheelbarrow,  the  drawing  out  of  bottle-corks,  and  the  gurgling  of  the 
flowing  liquor,  the  sound  of  air  rushing  through  a  crevice  on  a  wintry 


How  are  the  tones  varied  and  regulated?  Upon  what  does  tlie  manage- 
ment of  the  voice  depend  ?    What  is  ventriloquism  ? 


132 


NATURAL  PHILOSOPHY. 


4.  The  art  of  ventriloquism  was  not  unknown  to  the  an- 
cients ;  and  it  is  supposed  by  some  authors  that  the  famous 
responses  of  the  oracles  at  Delphi,  at  Ephesus,  &c.,  were  de- 
livered by  persons  who  possessed  this  faculty.  There  is  no 
doubt  that  many  apparently  wonderful  pieces  of  deception, 
which,  in  the  days  of  superstition  and  ignorance,  were  con- 
sidered as  little  short  of  miracles,  were  performed  by  means 
of  ventriloquism.  Thus  houses  have  been  made  to  appear 
haunted,  voices  have  been  heard  from  tombs,  and  the  dead 
have  been  made  to  appear  to  speak,  to  the  great  dismay  of 
the  neighborhood,  by  means  of  this  wonderful  art. 

5.  Ventriloquism  is,  without  doubt,  in  great  measure  the 
gift  of  nature ;  but  many  persons  can,  with  a  httle  practice, 
utter  sounds  and  pronounce  words  without  opening  the  lips  or 
moving  the  muscles  of  the  face  ;  and  this  appears  to  be  the 
great  secret  of  the  art. 


CHAPTER  IX. 

PYRONOMICS,  OR  THE  LAV^S  OF  HEAT. 

168.  Pyronomics  is  the  science  which  treats  of  the 
laws,  the  properties,  and  operations  of  heat.^ 

1.  The  nature  of  heat  is  unknown;  but  it  has  been  proved 
that  the  addition  of  heat  to  any  substance  produces  no  per- 
ceptible alteration  in  the  weight  of  that  substance.  Hence  it 
is  inferred  that  heat  is  imponderable. 

2.  Heat  pervades  all  bodies,  insinuating  itself,  more  or  less, 

night,  and  a  great  variety  of  other  noises  of  the  same  kind,  are  imitated  by 
the  voice  so  exactly,  as  to  deceive  any  hearer  vvho  does  not  know  whence 
they  proceed. 

*  Heat  is  undoubtedly  a  positive  substance  or  quality.  Cold  is  merely 
negative,  being  only  the  absence  of  heat 

Was  this  art  knov^^n  to  the  ancients  ?  What  is  supposed,  by  some  au- 
thors, concerning  the  responses  at  Delphi,  Ephesus,  &c.  ?  Is  ventriloquism 
a  natural  gift,  or  an  acquired  one  ? 

168.  What  is  Pyronomics?  What  is  said  in  regard  to  the  nature  of 
heat?    Is  it  ponderable  or  imponderable  ? 


PYRONOMICS. 


bet^^-een  their  particles,  and  forcing  them  asunder.  Heat,  and 
the  attraction  of  cohesion,  constantly  act  in  opposition  to  each 
otht.r  ;  hence  the  more  a  body  is  heated,  the  more  its  particles 
will  be  separated. 

3,  The  effect  of  heat  in  separating  the  particles  of  different 
kinds  of  substances  is  seen  in  the  melting  of  solids,  such  as 
metals,  wax,  butter,  &c.  The  heat  insinuates  itself  between 
the  particles,  and  forces  them  asunder.  These  particles  then 
are  removed  from  that  degree  of  proximity  to  each  other  with- 
in which  cohesive  attraction  exists,  and  the  body  is  reduced  to 
a  fluid  form.  When  the  heat  is  removed  the  bodies  return  to 
their  former  solid  state.* 

4 .  Heat  passes  through  some  bodies  with  more  difficulty 

*  Of  all  the  effects  of  heat,  that  produced  upon  water  is,  perhaps,  the 
mosl  remarkable.  The  particles  are  totally  separated  and  converted  into 
steam  or  vapor,  and  their  extension  is  wonderfully  increased.  The  steam 
which  arises  from  boiling  water  is  nothing  more  than  portions  of  the  water 
heated.  The  heat  insinuates  itself  between  the  particles  of  the  water,  and 
forces  them  asunder.  When  deprived  of  the  heat,  the  particles  will  unite 
in  th.e  form  of  drops  of  water.  This  fact  can  be  seen  by  holding  a  cold 
plate  over  boiling  water.  The  steam  rising  from  the  water  will  be  con- 
densed into  drops  on  the  bottom  of  the  plate.  The  air  which  we  breathe 
generally  contains  a  considerable  portion  of  moisture.  On  a  cold  day,  this 
moisture  condenses  on  the  glass  in  the  windows,  and  becomes  visible.  We 
see  it  also  collected  into  drops  on  the  outside  of  a  tumbler  or  other  vessel 
containing  cold  water  in  warm  weather.  Heat  also  produces  most  re- 
markable effects  upon  air,  causing  it  to  expand  to  a  wonderful  extent, 
while  the  absence  of  heat  causes  it  to  shrink  or  contract  into  very  small 
dimensions.  The  attraction  of  cohesion  causes  the  small  watery  particles 
which  compose  mist  or  vapor  to  unite  together  in  the  form  of  drops  of 
water.  It  is  thus  that  rain  is  produced.  The  clouds  consist  of  mist  or 
vapor  expanded  by  heat.  They  rise  to  the  cold  regions  of  the  skies,  where 
the  particles  of  vapor  lose  their  heat,  and  then,  uniting  in  drops,  fall  to  the 
earth.  But  so  long  as  they  retain  their  heat,  the  attraction  of  cohesion  can 
have  no  influence  upon  them,  and  they  will  continue  to  exist  in  the  form 
of  steam,  vapor,  or  mist. 


W^hat  effect  has  heat  upon  bodies?  What  two  forces  continually  act 
in  opposition  to  each  other?  In  what  can  the  effect  of  heat  be  seen?  How 
does  it  separate  the  particles?  What  would  be  the  effect  were  the  heat 
rem(ved  ?  Upon  what  has  heat  the  most  remarkable  effect  ?  How  does 
it  affect  it  ?  What  effect  has  heat  upon  air  ?  How  is  rain  produced  ? 
What  is  stated  with  regard  to  heat  ? 


134 


NATURAL  PHILOSOPHY. 


than  tlirougli  others  ;  but  there  is  no  kmd  of  matter  which  can 
compietely  arrest  its  progress.^ 

169.  The  principal  efFects  of  heat  are  three,  namely  : 

ist.  Heat  expands  most  substances. 

•2d.  It  converts  them  from  a  sohd  to  a  fluid  state. 

3d   It  destroys  their  texture  by  combustion.f 

*  The  thermometer,  an  iiistrmiient  designed  to  measure  degrees  of  heat, 
has  already  been  d.escribed,  in  connexion  with  the  barometer,  under  the 
head  of  Pneumatics.  Heat,  under  the  name  of  caloric,  is  properly  a  sub- 
ject of  consideration  in  the  science  of  Chemistr}^  It  exists  in  two  states, 
called,  respectively,  free  heat  and  latent  heat.  Free  heat,  or  free  caloric, 
is  that  which  is  perceptible  to  the  senses,  as  the  heat  of  a  fire,  the  heat  of 
the  sun,  &c.  Latent  heat  is  that  which  exists  in  most  kinds  of  substances, 
but  is  not  perceptible  to  the  senses,  until  it  is  brought  out  by  mechanical 
or  chemical  action.  Thus,  when  a  piece  of  cold  iron  is  hammered  upon 
an  anvil,  it  becomes  intensely  heated  ;  and  when  a  small  portion  of  sul- 
phuric acid,  or  vitriol,  is  poured  into  a  vial  of  cold  water,  the  vial  and  the 
hquid  immediately  become  hot.  A  further  illustration  of  the  existence  of 
latent  or  concealed  heat  is  given  at  the  fireside  every  day.  A  portion  of  cold 
fuel  is  placed  upon  the  grate  or  hearth,  and  a  spark  is  appUed  to  kindle  the 
fire  which  warms  us.  It  is  evident  that  the  heat  given  out  by  the  fuel, 
when  ignited,  does  not  all  proceed  from  the  spark,  nor  can  we  perceive  it 
in  the  fuel :  it  must,  therefore,  have  existed  somewhere  in  a  latent  state. 
It  is,  however,  the  effects  of  free  heat,  or  free  caloric,  which  are  embraced 
in  the  science  of  Pyronomics.  The  subject  of  latent  heat  belongs  more 
properly  to  the  science  of  Chemistry-. 

The  terms  heat  and  cold,  as  they  are  generally  used,  are  merely  rela- 
tive terms  :  for  a  substance  which  in  one  person  would  excite  the  sensation 
of  heat,  might,  at  the  same  time,  seem  cold  to  another.  Thus,  also,  to  the 
same  individual,  the  same  thing  may  be  made  to  appear,  relatively,  both 
warm  and  cold.  If,  for  instance,  a  person  were  to  hold  one  hand  near  to 
a  warm  fire,  and  the  other  on  a  cold  stone,  or  marble  slab,  and  then  plunge 
both  into  a  basin  of  lukewarm  water,  the  liquid  would  appear  cold  to  the 
vrarm  hand  and  warm  to  the  cold  one. 

t  These  eiiects  do  not  take  place  ia  all  substances.    Some  substances 


Can  the  progress  of  heat  be  arrested  ?  What  is  caloric  ?  In  what  two 
states  does  heat  exist  ?  What  is  free  heat  ?  Give  some  examples  of  free 
heat.  What  is  latent  heat  ?  Give  some  examples  of  latent  heat.  How 
are  the  terms  heat  and  cold  generally  used  ■  What  illustration  of  this  is 
given  ? 

1G9.  What  are  the  three  principal  effects  of  heat  on  bodies  to  which  it 
is  applied?    Give  an  example  of  each  effect. 


PYRONOMICS. 


135 


170.  Heat  tends  to  diffuse  itself  equally  through  all 
substances. 

1.  If  a  heated  body  be  placed  near  a  cold  one,  the  tempera- 
ture of  the  former  will  be  lowered,  while  that  of  the  latter  will 
be  raised. 

2.  All  substances  contain  a  certain  quantity  of  heat ;  but, 
on  account  of  its  tendency  to  diffuse  itself  equally,  and  the 
difference  in  the  power  of  different  substances,  to  conduct  it, 
bodies  of  the  same  absolute  temperature  appear  to  possess 
different  degrees  of  heat. 

3.  Thus,  if  the  hand  be  successively  applied  to  a  woollen 
garment,  a  mahogany  table,  and  a  marble  slab,  all  of  which 
have  stood  for  some  time  in  the  same  l  Oom,  the  woollen  gar- 
ment will  appear  the  warmest,  and  the  marble  slab  the  coldest 
of  the  three  articles  ;  but  if  a  thermometer  be  applied  to  each, 
ao  difference  in  the  temperature  will  be  observed. 

4.  From  this  it  appears,  that  some  substances  conduct  heat 
readily,  and  others  with  great  difficulty.  The  reason  that  the 
marble  slab  seems  the  coldest,  is,  that  marble,  being  a  good 
conductor  of  heat,  receives  the  hccat  from  the  hand  so  readily 
that  the  loss  is  instantly  felt  by  the  hand  ;  w^hile  the  woollen 
garment,  being  a  bad  conductor  of  heat,  receives  the  heat 
from  the  hand  so  slowly  that  the  loss  is  imperceptible. 

171.  The  different  power  of  receiving  and  conducting 
heat,  possessed  by  different  substances,  is  the  cause  of 
the  difference  in  the  warmth  of  various  substances  used 
for  clothing. 

y,re  incombustible  ;  others  cannot  be  transformed  to  a  fluid  state  by  any 
degree  of  heat  yet  produced  artificially.    The  expansive  effect  of  heat  has 
but  one  known  exception.    The  sources  from  which  heat  is  derived  are — 
1st.  From  the  sun  in  connexion  with  light ; 

2dly.  From  mechanical  operations,  such  as  friction,  percussion,  and 
compression  ; 

3dly.  From  chemical  operations,  especially  combustion ; 
4thly.  From  hving  animals  and  vegetables. 

What  are  the  sources  of  heat  ? 

170.  In  what  way  does  heat  tend  to  diffuse  itself?  Why  do  bodies  of 
the  same  absolute  temperature  appear  to  possess  different  degrees  of  heat  ? 
Wliat  illustration  of  this  is  given  ?    What  appears  from  this? 

171.  What  causes  the  difference  in  the  warmth  of  substances  used  for 
clothing? 


NATURAL  PHILOSOPHY. 


1.  Thus,  woollen  garments  are  warm  garments,  because  they 
part  slowlv  with  the  heat  which  they  acquire  from  the  body, 
and,  consequently,  they  do  not  readily  convey  the  warmth  of 
the  body  to  the  air ;  while,  on  the  contrary,  a  linen  garment  is 
a  cool  one,  because  it  parts  with  its  heat  readily,  and  as  readi- 
ly receives  fresh  heat  from  the  body.  It  is,  therefore,  con- 
stantly receiving  heat  from  the  body  and  throwing  it  out 
into  the  air,  while  the  woollen  garment  retains  the  heat  which 
it  receives,  and  thus  encases  the  body  with  a  warm  covering. 

2.  For  a  similar  reason  ice,  in  summer,  is  wrapped  in  woollen 
cloths.  It  is  then  protected  from  the  heat  of  the  air,  and  will 
not  melt. 

172.  Heat  is  propagated  in  two  ways,  namely,  by 
conduction  and  by  radiation.  Heat  is  propagated  by 
conduction  when  it  passes  from  one  substance  to  another 
in  contact  with  it.  Heat  is  propagated  by  radiation 
when  it  passes  through  the  air  or  any  other  elastic 
fluid. 

173.  Different  bodies  conduct  heat  with  different  de- 
grees of  facility.  The  metals  are  the  best  conductors, 
and  among  metals  silver  is  the  best  conductor. 

1.  For  this  reason  any  hquid  may  be  heated  in  a  silver  vessel 
more  readily  than  in  any  other  of  the  same  thickness.  The 
metals  stand  in  the  following  order,  with  respect  to  their  con- 
ducting power ;  namely,  silver,  gold,  tin,  copper,  platina,  steel, 
iron,  and  lead. 

2.  It  is  on  account  of  the  conducting  power  of  metals^  that 

*  Metals,  on  account  of  their  conducting  power,  cannot  be  handled 
when  raised  to  a  temperature  above  120  degrees  of  Fahrenheit.  Water 
becomes  scalding  hot  at  150  degrees,  but  air,  heated  far  beyond  the  tem- 
perature of  boiling  water,  may  be  applied  to  the  skin  without  much  pain. 
Sir  Joseph  Banks,  with  several  other  gentlemen,  remained  some  time  in  a 
room  when  the  heat  was  52  degrees  above  the  boiling  point ;  but,  though 
they  could  bear  the  contact  of  the  heated  air,  they  could  not  touch  any 
metallic  substance,  as  their  watch-chains,  money,  &c.    Eggs,  placed  on 


172.  In  what  two  ways  is  heat  propagated?  When  is  it  propagated  by 
conduction?    When  is  it  propagated  by  radiation? 

173.  Do  all  bodies  conduct  heat  with  the  same  degree  of  facility  ?  What 
bodies  are  the  best  conductors?  In  what  order  do  the  metals  stand  with 
respect  to  their  conducting  power'? 


I'YRONOMICS. 


137 


the  handles  of  metal  tea-pots  and  coffee-pots  are  commonly 
made  of  wood  ;  since,  if  they  were  made  of  metal,  they  would 
become  too  hot  to  be  grasped  by  the  hand,  soon  after  the  vessel 
is  filled  with  heated  fluid.  Wood  conducts  heat  very  imper- 
fectly. For  this  reason  wooden  spoons  and  forks  are  preferred 
for  ice.  Indeed,  so  imperfect  a  conductor  of  heat  is  wood,  that 
a  stick  of  wood  may  be  grasped  by  the  hand  while  one  end  of 
the  stick  is  a  burning  coal.  Animal  and  vegetable  substances, 
of  a  loose  texture,  such  as  fur,  wool,  cotton,  &c.,  conduct  heat 
very  imperfectly  ;  hence  their  efficacy  in  preserving  the  warmth 
of  the  body. 

174.  Heat  is  reflected  from  bright  surfaces;  while 
black  or  dark  colored  bodies  absorb  the  heat  that  falls 
on  them. 

1.  This  is  the  reason  why  the  bright  brass  andirons,  or  any 
other  bright  substances,  placed  near  a  hot  fire,  seldom  become 
heated  ;  while  other  dark  substances,  further  removed  from  the 
fire,  become  too  hot  for  the  hand. 

2.  Snow  or  ice  will  melt  under  a  piece  of  black  cloth,  when 
it  will  remain  perfectly  sohd  under  a  white  one.  The  farmers 
in  some  of  the  mountainous  parts  of  Europe,  are  accustomed 
to  spread  black  earth,  or  soot,  over  the  snow,  in  the  spring,  to 
hasten  its  melting,  and  enable  them  to  commence  ploughing 
early. 

175.  All  bodies,  when  violently  compressed  or  ex- 
tended, become  warm. 

a  till  frame,  were  roasted  hard  in  twenty  minutes  ;  and  a  beef-steak  was 
overdone  in  thirty-three  minutes. 

Chantrey,  the  celebrated  sculptor,  had  an  oven  which  he  used  for  drying 
his  piaster  cuts  and  moulds.  The  thermometer  generally  stood  at  300  de- 
grees in  it,  yet  the  workmen  entered,  and  remained  in  it  some  minutes 
without  difficulty  ;  but  a  gentleman  once  entering  it  with  a  pair  of  silver- 
mounted  spectacles  on,  had  his  face  burnt  where  the  metal  came  in  con- 
tact with  the  skin. 

Is  wood  a  good  conductor  of  heat  ?  Why  are  wool,  fur,  &c.,  so  effi- 
cacious in  preserving  the  warmth  of  the  body?  What  is  related  in  the 
note  with  regard  to  the  conducting  power  of  heat? 

174.  What  bod.es  reflect  the  heat?  What  bodies  absorb  the  heat"? 
Why  do  bright  bodies,  when  placed  near  the  fire,  seldom  become  heated? 
Will  snow  melt  most  readily  under  white  or  black  cloth  ? 

175.  What  effect  is  produced  on  all  bodies  when  violently  compressed 
or  extended  ? 


138 


NATURAL  PHILOSOPHY. 


1.  If  a  piece  of  india-rubber  be  quickly  stretched  and  ap- 
plied to  the  lip,  a  sensible  degree  of  heat  will  be  felt.  An  iron 
bar,  on  being  hammered,  becomes  red-hot ;  and  even  water, 
when  strongly  compressed,  gives  out  heat. 

2.  When  air  is  forcibly  compressed  by  driving  down  the 
piston  of  a  syringe,  nearly  closed  at  the  end,  great  heat  is  pro- 
duced. Syringes  have  been  constructed  on  this  principle  for 
procuring  fire,  the  heat,  thus  produced,  being  sufficient  to 
kindle  dry  tinder. 

176.  All  substances,  as  they  are  affected  by  heat, 
may  be  divided  into  combustible  and  incombustible 
bodies.* 

177.  The  pyrometerf  is  an  instrument  to  show  the 
expansion  of  bodies  by  the  application  of  heat. 

It  consists  of  a  metallic  bar  or  wire,  with  an  index  connected 
with  one  extremity.  On  the  application  of  heat  the  bar  ex- 
pands and  tui-ns  the  index  to  show  the  degree  of  expansion. 

178.  The  most  obvious  and  direct  effect  of  heat  on  a 
body,  is  to  increase  its  extension  in  all  directions. 

1.  Coopers,  wheelwrights,  and  other  artificers,  avail  them- 
selves of  this  property  in  fixing  iron  hoops  on  casks,  and  the 
tires  or  irons  on  wheels.  The  hoop  or  tire  having  been  heated, 
expands,  and  being  adapted  in  that  state  to  the  cask  or  the 
wheel,  as  the  metal  contracts  in  cooling,  it  clasps  the  parts, 
very  firmly  together.]; 

*  Vegetable  substances,  charcoal,  oils,  most  animal  substances,  as  hair, 
wool,  horn,  fat,  and  all  metallic  bodies,  are  combustible.  Stones,  glass, 
salts,  &LC.,  are  incombustible. 

t  Wedge  wood's  pyrometer,  the  instrument  commonly  used  for  high 
temperatures,  measures  heat  by  the  contraction  of  clay. 

t  From  what  has  been  stated  above,  it  will  be  seen,  that  an  allowance 
should  be  made  for  the  alteration  of  the  dimensions  in  metallic  beams  or 


"What  experiments  are  here  related  to  iliustrdte  this  ?  What  is  said  oi 
the  air  when  strongly  compressed  1 

176.  Into  what  classes  are  all  substances,  as  affected  by  heat,  divided  ? 
What  substances  are  combustible  ?    What  substances  are  incombustible  ? 

177.  What  is  a  pyrometer?  Of  what  does  it  consist  ?  How  does  Wedge- 
wood's  pyrometer  measure  high  temperatures? 

178.  What  is  the  most  obvious  and  direct  effect  of  heat  on  a  body 
What  application  of  this  principle  is  related  in  the  note  ? 


PYRONOMICS. 


130 


2.  The  effect  of  heat  and  cold,^  in  the  expansion  and  con- 
traction of  glass,  is  an  object  of  common  observation;  for  it  is 
tins  expansion  and  contraction  which  cause  so  many  accideats 
wilh  glass  articles.  Thus,  when  hot  water  is  suddenly  poured 
inlo  a  cold  glass,  of  any  form,  the  glass,  if  it  have  any  thick- 
ness, will  crack  ;  and,  on  the  contrary,  if  cold  water  be  pou:  td 
into  a  heated  glass  vessel,  the  same  effect  will  be  produced. 
The  reason  of  \vhich  is  this  :  heat  makes  its  way  but  slowly 
through  glass  ;  the  inner  surface,  therefore,  when  the  hot 
water"^  is  poured  into  it,  becomes  heated,  and,  of  course,  dis- 
tended before  the  outer  surface,  and  the  irregular  expansion 
causes  the  vessel  to  break.  There  is  less  danger  of  fracture, 
therefore,  when  the  glass  is  thin,  because  the  heat  readily 
penetrates  it,  and  there  is  no  irregular  expansion.j 

supporters,  caused  by  the  dilatation  and  contraction  effected  by  the  weather. 
In  the  iron  arches  of  Southwark  bridge,  over  the  Thames,  the  variation  of 
the  temperature  of  the  air  causes  a  difference  of  height,  at  different  times, 
amounting  to  nearly  an  inch.  A  happy  apphcation  of  this  principle  to  the 
mechanic  arts  was  made,  some  years  ago,  at  Paris.  The  weight  of  the 
roof  of  a  building,  in  the  Conservatory  of  Arts  and  Trades,  had  pressed 
outwards  the  side  walls  of  the  structure,  and  endangered  its  security.  The 
following  method  was  adopted  to  restore  the  perpendicular  direction  of  the 
structure.  Several  apertures  were  made  in  the  walls,  opposite  to  each 
other,  through  which  iron  bars  were  introduced,  which,  stretching  across 
the  building,  extended  beyond  the  outside  of  the  walls.  These  bars  ter- 
minated in  screws,  at  each  end,  to  which  large  broad  nuts  were  attached. 
Each  alternate  bar  was  then  heated  by  means  of  powerful  lamps,  and 
their  lengths  being  thus  increased,  the  nuts  on  the  outside  of  the  building 
were  screwed  up  close  to  it,  and  the  bars  were  suffered  to  cool.  The  pow- 
erful contraction  of  the  bars  drew  the  walls  of  the  building  closer  together, 
and  the  same  process  being  repeated  on  all  the  bars,  the  walls  were  grad- 
ually and  steadily  restored  to  their  upright  position. 

*  Cold  is  merely  the  absence  of  heat ;  or  rather,  more  properly  speak- 
ing, inferior  degrees  of  heat  are  termed  cold. 

t  The  glass  chimneys,  used  for  oil  and  gas  burners,  are  often  broken  by 
being  suddenly  placed,  when  cold,  over  a  hot  flame.  The  danger  of  frac- 
ture may  be  prevented  (it  is  said)  by  making  a  minute  notch  on  the  bot- 
tom of  the  tube  with  a  diamond.    This  precaution  has  been  used  in  an 


What  is  said  of  the  effect  of  heat  and  cold  on  glass?  When  hot  water 
is  suddenly  poured  into  a  cold  glass,  why  will  the  glass  crack?  When 
cold  water  is  applied  to  a  heated  glass,  why  will  the  glass  crack? 


140 


NATURAL  PHILOSOPHY. 


179.  The  expansion  caused  by  heat  in  soHd  and 
liquid  bodies  differs  in  different  substances  ;  but  aeri- 
form fluids  all  expand  alike,  and  undergo  uniform  de- 
grees of  expansion  at  various  temperatures. 

The  expansion  of  solid  bodies  depends,  in  some  degree,  on 
the  cohesion  of  their  particles  ;  but  as  gases  and  vapors  are 
destitute  of  cohesion,  heat  operates  on  them  without  any  op- 
posing power. 

180.  The  density  of  all  substances  is  augmented  by 
cold,  and  diminished  by  heat. 

There  is  a  remarkable  exception  to  this  remark,  and  that  is 
in  the  case  of  water ;  which,  instead  of  contracting,  expands  at 
the  freezing  point,  or  when  it  is  frozen.  This  is  the  reason  why 
pitchers,  and  other  vessels,  containing  water  and  other  similar 
fluids,  are  so  often  broken  when  the  hquid  freezes  in  them. 
For  the  same  reason,  ice  floats^  instead  of  sinking  in  water ; 
for  as  its  density  is  diminished,  its  specific  gravity  is  conse- 
quently diminished. 

181.  Different  bodies  require  different  quantities  of 
heat  to  raise  them  to  the  same  temperature ;  and  those 

establishment  where  six  lamps  were  lighted  every  day,  and  not  a  single 
glass  has  been  broken  in  nine  years. 

*  Were  it  not  for  this  remarkable  property  of  water,  large  ponds  and 
lakes,  exposed  to  intense  cold,  would  become  solid  masses  of  ice  ;  for  if  the 
ice,  when  formed  on  the  surface,  were  more  dense  (that  is,  more  heavy) 
than  the  water  below,  it  would  sink  to  the  bottom,  and  the  water  above, 
freezing  in  its  turn,  would  also  sink,  until  the  whole  body  of  the  water  would 
be  frozen.  The  consequence  would  be  the  total  destruction  of  all  crea- 
tures in  the  water.  But  the  specific  gravity  of  ice  causes  it  to  continue  on 
the  surface,  protecting  the  water  below  from  congelation. 


179.  Is  the  expansion  caused  by  heat  in  solid  and  liquid  bodies  the  same 
in  all  substances  ?  How  do  aeriform  fluids  differ,  in  this  respect,  from 
solid  and  liquid  bodies  ?  Upon  what  does  the  expansion  of  solid  bodies  in 
some  degree  depend?    Why  has  heat  more  power  over  gases  and  vapors? 

180.  What  effect  has  heat  and  cold  upon  the  density  of  all  substances? 
What  exception  is  there  to  this  remark  ?  Why  are  the  vessels,  contain- 
ing water  and  other  similar  fluids,  so  often  broken  when  the  liquid  freezes 
in  them?  Why  does  ice  float  upon  the  water,  instead  of  sinking  in  it? 
What  is  stated  in  the  note  with  regard  to  this  property  of  v/ater  ? 


I'VKOXOMICS 


which  iwc  hiMl'Ml  with  mosl  (hllicuhy  rcUuii  their  heat 
the  loiii^est. 

Thus  oil  hei'onios  heated  more  s])ee(lily  tlian  water,  and  it 
hkewise  cools  more  (juickly. 

18\2.  When  heat  is  thrown  upon  a  bright  or  polished 
surface  it  is  reflected,*  and  the  angle  of  reflection  will 
he  eipial  to  the  angle  of  incidence. 

18:^.  When  a  certain  degree  of  heat  is  applied  to 
water  it  converts  it  into  steam  or  vapor. 

184.  The  temperature  of"  steam  is  always  the  same 
with  that  of  the  liquid  from  which  it  is  formed,  while  it 
remains  in  contact  with  that  liquid.  When  closely  con- 
fined, its  elastic  power  is  often  sufficient  to  burst  the 
vessel  in  which  it  is  confined. 

185.  The  elastic  force  of  steam  is  increased  by  heat, 
and  diminished  by  cold.  The  amount  of  pressure, 
therefore,  which  it  will  exert  depends  on  the  tempera- 
ture at  which  it  is  formed. 

186.  The  great  and  peculiar  property  of  steam,  on 
which  its  mechanical  agencies  d^epend,  is  its  power  of 

*  Advantage  has  been  taken  of  this  property  of  heat  in  the  construction 
of  a  simple  apparatus  for  baking.  It  is  a  bright  tin  case,  having  a  cover 
incHned  towards  the  fire  in  such  a  manner  as  to  reflect  the  heat  down- 
wards. In  this  manner  use  is  made  both  of  the  direct  heat  of  the  fire,  and 
the  reflected  heat,  which  would  otherwise  pass  into  the  room.  The  whole 
apparatus,  thus  connected  with  the  culinary  department,  is  called,  in  New 
England,  "  The  Connecticut  baker" 


181.  Can  all  bodies  be  raised  to  the  same  temperature  by  the  same 
quantities  of  heat?    What  bodies  retain  their  heat  the  longest? 

182.  What  becomes  of  the  heat  which  is  thrown  upon  a  bright  or  pol- 
ished surface  ?  How  do  the  angles  of  incidence  and  reflection  compare 
with  each  other? 

183.  When  is  water  converted  into  steam  or  vapor  ? 

184.  How  does  the  temperature  of  the  stearn  compare  with  that  of  the 
liquid  from  which  it  is  formed  while  it  remains  in  contact  with  that  liquid? 

185.  By  what  is  the  elasticity  of  steam  increased  and  diminished  ? 
Upon  what  does  the  amount  of  pressure,  which  steam  exerts,  depend  ? 

186.  What  is  the  great  and  peculiar  property  of  steam,  on  which  its 
mechanical  agencies  depend? 


142 


NATURAL  PHILOSOPHY. 


exerting  a  high  degree  of  elastic  force,  and  losing  it  in- 
stantaneously. 

187.  The  steam-engine  is  a  machine  moved  by  the 
expansive  force  of  steam.* 

188.  Steam  occupies  a  space  about  1700  times  larger 
than  it  will  when  converted  into  water.  If,  therefore, 
the  steam  in  a  cylinder  be  suddenly  converted  into 
water,  it  will  occupy  a  much  smaller  space,  and  pro- 
duce a  vacuum  in  the  cylinder. 

1.  The  mode  in  which  steam  is  made  to  act,  is  by  causing 
its  expansive  force  to  raise  a  solid  piston  accurately  fitted  to 
the  bore  of  a  cylinder,  like  that  in  the  forcing-pump. 

2.  The  piston-rod  rises  by  the  impulse  of  expanding  steam, 
admitted  into  the  cylinder  below.  When  the  piston  is  thus 
raised,  if  the  steam  below  it  be  suddenly  condensed,  or  with- 
drawn from  under  it,  a  vacuum  will  be  formed,  and  the  pres- 
sure of  the  atmosphere  on  the  piston  above  will  drive  it  down. 
The  admission  of  more  steam  below  it  Avill  raise  it  again,  and 
thus  a  continued  motion  of  the  piston,  up  and  down,  will  be 
produced.  This  motion  of  the  piston  is  communicated  to  wheels, 
levers,  and  other  machinery,  in  such  a  manner  as  to  produce 
the  effect  intended.f 

3.  The  celebrated  Mr.  James  Watt  introduced  two  important 
improvements  into  the  steam-engine.  Obser\ang  that  the 
cooling  of  the  cylinder  by  the  water  thrown  into  it  to  condense, 
the  st'eam,  lessened  the  expansibility  of  the  steani ;  he  con- 
trived a  method  to  withdraw  the  steam  from  the  principal  cyl- 
inder, after  it  had  performed  its  office,  into  a  condensing- 

*  Steam,  as  it  issues  into  the  air,  is  visible,  and  resembles  smoke  in  itt 
appearance,  because  the  coldness  of  the  air  instantly  condenses  it  intc 
minute  watery  globules  ;  but  while  performing  its  office,  it  is  perfectly  dry, 
that  is,  it  contains  no  watery  particles,  but  is  expanded  into  so  rare  a  state 
as  to  be  absolutely  invisible. 

t  This  is  the  mode  in  which  the  engine  of  Newcomen  and  Savery, 
commonly  called  the  atmospheric  engine,  was  constructed. 


187.  What  is  the  steam-engine? 

188.  How  much  larger  space  does  steam  occupy  than  water?  By 
what  mode  is  steam  made  to  act  ?  By  what  impulse  does  the  piston  rise? 
What  causes  the  piston  to  descend?  AVhat  improvement  did  Mr  Watt 
introduce  into  the  steam-engine  ? 


rVRONOMICS. 


143 


cliamber,  where  it  is  reconverted  into  water,  and  conveyed 
back  to  the  boiler. 

4.  The  otlicr  improvement  consists  in  substituting  the  ex- 
pansive power  of  steam  for  the  atmospheric  pressure.  This 
was  performed  by  admitting  the  steam  into  the  cyhnder  ahove 
the  raised  piston,  at  the  same  moment  that  it  is  removed  from 
heloio  it ;  and  thus  the  power  of  steam  is  exerted  in  the  descend- 
ing as  well  as  in  the  ascending  stroke  of  the  piston ;  and  a 
much  greater  impetus  is  given  to  the  machinery  than  by  the 
former  method.  From  the  double  action  of  the  steam  ahove, 
as  well  as  below  the  piston,  and  from  the  condensation  of  the 
steam,  after  it  has  performed  its  office,  this  engine  is  called 
Watt's  double-acting  condensing  steam-engine. 

5.  Fig.  93  represents  that  portion  of  the  steam-engine  in 
which  steam  is  made  to  act,  and  propel  such  machinery  as  may 
be  connected  with  it.  The  principal  parts  are  the  boiler,  the 
cylinder  and  its  piston, 
the  condenser,  the  air- 
pump,  the  steam-pipe, 
the  eduction-pipe,  and 
the  cistern.  In  this  figure, 
A  represents  the  boiler, 
C  tht  cylinder,  with  H 
the  piston,  B  the  steam- 
pipe,  with  two  branches^ 
communicating  with  the 
cylinder,  the  one  above 
and  the  other  below  the 
piston.  This  pipe  has  two  valves,  F  and  G,  which  are  opened 
and  closed  alternately  by  machinery  connected  with  the  piston. 
The  steam  is  carried  through  this  pipe  by  the  valves,  w^hen 
open,  to  the  cylinder  both  above  and  below  the  piston.  K  is 
the  eduction-pipe,  having  two  branches,  like  the  steam-pipe, 
furnished  with  valves,  &c.,  which  are  opened  and  shut  by  the 
same  machinery.  By  the  eduction-pipe  the  steam  is  led  off 
from  the  cylinder  as  the  piston  ascends  and  descends. 

*  The  steam  and  the  eduction  pipes  are  sometimes  made  in  forms  dif- 
fering from  those  in  the  figure,  and  they  differ  much  in  different  engines. 


What  does  Fig.  93  represent?  What  are  the  principal  parts?  What 
does  A  represent?  What  does  C  represent?  What  does  B  represent? 
What  does  K  represent?   By  what  is  the  steam  led  ofTfrom  the  cylinder? 


Fig.  93. 


X 

J 


144 


NATURAL  PHILOSOPHY. 


L  is  the  condenser,  and  O  a  stop-cock  for  the  admission  of 
cold  water.  M  is  the  air-pump.  N  is  the  cistern  of  cold  water 
in  which  the  condenser  is  immersed.  R  is  the  safety-valve. 
When  the  valves  are  all  open,  the  steam  issues  freely  from  the 
boiler,  and  circulates  through  all  the  parts  of  the  machine,  ex- 
pelling the  air.*  Now,  the  valves  F  and  Q,  being  closed,^  and 
G  and  P  remaining  open,  the  steam  presses  upon  the  piston - 
and  forces  it  down.  As  it  descends,  it  draws  with  it  the  end 
of  the  working-beam,  which  is  attached  to  the  piston-rod  J, 
(but  which  is  not  represented  in  the  figure.)  To  this  working- 
beam,  (which  is  a  lever  of  the  first  kind,)  bars  or  rods  are  at- 
tached, which,  rising  and  falling  with  the  beam  and  the  piston, 
open  the  stop-cock  0,  admitting  a  stream  of  cold  water,  which 
meets  the  steam  from  the  cylinder  and  condenses  it,  leaving  no 
force  below  the  piston  to  oppose  its  descent.  At  this  moment 
the  rods  attached  to  the  working-beam  close  the  stop-cocks  G 
and  P,  and  open  F  and  Q.  The  steam  then  flows  in  below  the 
piston,  and  rushes  from  above  it  into  the  condenser,  by  which 
means  the  piston  is  forced  up  again  with  the  same  power  as 
that  with  which  it  descended.  Thus  the  steam-cocks  G  and 
P  and  F  and  Q  are  ahernately  opened  and  closed ;  the  steam 
passing  from  the  boiler  drives  the  piston  ahernately  upwards 
and  downwards,  and  thus  produces  a  regular  and  continued 
motion.  This  motion  of  the  piston,  being  communicated  to  the 
working-beam,  is  extended  to  other  machinery,  and  thus  an 
engine  of  great  power  is  obtained. 

The  air-pump  M,  the  rod  of  which  is  connected  with  the 
working-beam,  carries  the  water  from  the  condenser  back  into 
the  boiler,  by  a  communication  represented  in  Fig.  94. 

*  This  process  is  called  blowing  out,  and  is  heard  when  a  steamboat  is 
about  starting. 


What  does  L  represent  ?  What  does  O  represent  ?  What  does  M  rep 
resent?  What  does  N  represent?  What  does  R  represent?  When  the 
valves  are  all  open,  what  becomes  of  the  steam  ?  When  the  valves  F  and 
Q  are  closed,  and  G  and  P  open,  upon  what  does  the  steam  press?  What 
does  the  cylinder  draw  with  it  in  its  descent?  Which  of  the  mechanical 
powers  is  this  working-beam  ?  What  are  attached  to  this  working-beam  ? 
What  is  their  use?  What  becomes  of  the  steam  when  the  stop-cocks  G 
and  P  are  closed,  and  F  and  Q  are  open?  How  is  the  regular  and  con- 
tinued motion  produced?  To  what  is  this  motion  of  the  piston  communi- 
cated ]  What  IS  the  use  of  the  air-pump  M  ?  For  what  is  the  safety- 
valve  R  used  ? 


PYRONOMICS. 


The  safety-valve  R,  connected  with  a  lever  of  the  second 
^ind,  is  made  to  open  v^hen  the  pressuie  of  the  steam  within 
the  boiler  is  too  great.  The  steam  then  rushing  through  the 
aperture  under  the  valve,  removes  the  danger  of  the  bursting 
of  the  boiler. 

189.  The  power  of  a  steam-engine  is  generally  ex- 
pressed by  the  power  of  a  horse,  which  can  raise 
33,000  lbs.  to  the  height  of  .one  foot  in  a  minute.  An 
engine  of  100  horse  power,  is  one  that  will  raise 
3,300,000  lbs.  to  the  height  of  one  foot  in  one  minute. 

190.  The  steam-engine*  is  constructed  in  various 
forms  ;  the  principal  of  which  are  the  high  and  the  low 
pressure  engines  ;  or,  as  they  are  sometimes  called,  the 
non-condensing  and  the  condensing  engines. 

1.  The  non-condensing  or  high-pressure  engines  differ  from 
the  low  pressure  or  condensing  engines  in  having  no  condenser. 
The  steam,  after  having  moved  the  piston,  is  let  off  into  the 
open  air.  As  this  kind  of  engine  occupies  less  space,  and  is 
much  less  compUcated,  it  is  generally  used  on  railroads. 

2.  In  the  low  pressure  or  condensing  engines,  the  steam,  after 
having  moved  the  piston,  is  condensed,  or  converted  into  water, 
and  then  conducted  back  into  the  boiler. 

*  The  steam-engine,  as  it  is  constructed  at  the  present  day,  is  the  re- 
sult of  the  inventions  and  discoveries  of  a  number  of  distinofuished  indi- 
viduals, at  different  periods.  Among  those  who  have  contributed  to  its 
present  state  of  perfection,  and  its  application  to  practical  purposes,  may 
be  mentioned  the  names  of  Somerset,  the  Marquis  of  Worcester,  Savery, 
Newcomen,  Fulton,  and  especially  Mr.  James  Watt. 

To  the  inventive  genius  of  Watt,  tiie  engine  is  indebted  for  the  con- 
denser ^  the  appendages  for  parallel  motion,  tiie  application  of  the  governor, 
and  for  the  double  action.  In  the  words  of  Mr.  Jeffrey  it  may  be  added, 
that,  "  by  his  admirable  contrivances,  and  those  of  Mr.  Fulton,  it  has  be- 
come a  thing  alike  stupendous  for  its  force  and  its  flexibility  ;  for  the  pro- 


189.  How  is  the  power  of  a  steam-engine  expressed?  What  is  an  en- 
gine of  100  horse  power  I 

190.  What  are  the  principal  forms  in  which  the  steam-engine  is  con- 
structed? How  do  they  diffei  from  each  other?  What  becomes  of  the 
steam  after  having  moved  the  piston  in  the  non-condensing  engines  ? 
What  kind  of  engines  is  generally  used  on  railroads?  What  becomes  of 
the  steam  after  having  moved  the  piston  in  the  condensing  engines? 

7 


146  NATURAL  PHILOSOPHY. 

watt's  DOUBLE' acting  CONDENSING  STEAM-ENGINE. 
Fig.  94. 


3.  Fig.  94  represents  Watt's  double-acting  condensing  steam- 
engine,  in  which  A  represents  the  boiler,  containing  a  large 
quantity  of  water,  which  is  constantly  replaced  as  fast  as  por- 
tions are  converted  into  steam.  B  is  the  steam-pipe,  convey- 
ing the  steam  to  the  cylinder,  having  a  steam-cock  h  to  admit 
or  exclude  the  steam  at  pleasure. 

C  is  the  cyhnder,  surrounded  by  the  jacket  cc,  a  space  kept 
constantly  supplied  with  hot  steam,  in  order  to  keep  the 
cylinder  from  being  cooled  by  the  external  air.    D  is  the 

digious  power  it  can  exert,  and  the  ease  and  precision  and  ductility  with 
which  it  can  be  varied,  distributed,  and  applied.  The  trunk  of  an  ele- 
phant, that  can  pick  up  a  pin,  or  rend  an  oak,  is  as  nothing  to  it.  It  can 
engrave  a  seal,  and  crush  masses  of  obdurate  metal  before  it :  draw  out, 
without  breaking,  a  thread  as  fine  as  gossamer,  and  lift  up  a  ship  of  war 
like  a  bauble  in  the  air.  It  can  embroider  muslin,  and  forge  anchors  ;  cut 
steel  into  ribands,  and  impel  loaded  vessels  against  the  fury  of  the  winds 
and  waves," 


rV  llONOiMlCS. 


117 


0(lucti()n-pij)(\  coniiniinicatiiiL;-  belwccii  tlic.  cylinder  nrul  tlio 
coiuloiiscr.  K  is  (lie  condenser,  with  a  valve  c,  calked  tlie 
injection-cock,  admitting  ii  jet  of  cold  water,  which  meets  the 
si  earn  the  instant  that  the  steam  enters  the  condenser.  F  is 
tl\e  air-pump,  which  is  a  common  suction-pump,  but  is  heie 
called  tlie  air-pump  because  it  removes  from  the  condenser  not 
only  the  water,  but  also  the  air,  and  the  steam  that  es(^apcs 
condensation.  G  G  is  a  cold-water  cistern,  which  surrounds 
the  condenser,  and  supplies  it  with  cold  water,  being  filled, 
by  the  cold- water  pump,  which  is  represented  by  H.  I  is  the 
hot  well,  containing  water  from  the  condenser.  K  is  the  hot- 
water  pump,  which  conveys  back  the  water  of  condensation 
from  the  hot  well  to  the  boiler. 

L  L  are  levers,  which  open  and  shut  the  valves  in  the  chan- 
nel between  the  steam-pipe,  cylinder,  eduction-pipe,  and  con- 
denser ;  which  levers  ai-e  raised  or  depressed  by  projections 
attached  to  the  piston-rod  of  the  pump.  M  M  is  an  apparatus 
for  changing  the  circular  motion  of  the  working-beam  into 
parallel  motion,  so  that  the  piston-rods  are  made  to  move  in  a 
straight  line.  N  N  is  the  working-beam,  which  being  moved 
by  the  rising  and  falhng  of  the  piston,  attached  to  one  end, 
communicates  motion  to  the  fly-wheel  by  means  of  the  crank 
P,  and  from  the  fly-wheel  the  motion  is  communicated  by 
bands,  wheels,  or  levers,  to  the  other  parts  of  the  machinery. 
0  O  is  the  governor. 

The  governor  being  connected  with  the  fly-wheel,  is  made 
to  participate  the  common  motion  of  the  engine,  and  the  balls 
will  remain  at  a  constant  distance  from  the  perpendicular 
shaft,  so  long  as  the  motion  of  the  engine  is  uniform ;  but 
whenever  the  engine  moves  faster  than  usual,  the  balls  will  re- 
cede farther  from  the  shaft,  and,  by  raising  a  valve  connected 
with  the  boiler,  will  let  ofl^  such  a  portion  of  the  force  as  to 
reduce  the  speed  to  the  rate  required. 

The  steam-engine,  thus  constructed,  is  apphed  to  boats  to 
turn  wheels  having  paddles  attached  to  their  circumference, 
which  answer  the  purpose  of  oars.    It  is  used  also  in  work- 


What  does  Fig.  94  represent  ?  What  does  A  represent  ?  What  does  B 
represent?  What  does  G  represent?  What  does  D  represent?  What 
does  E  represent?  What  does  F  represent?  What  does  G  G  represent? 
What  does  I  represent?  What  does  K  represent?  What  does  LL  rep- 
resent? What  does  M  M  represent?  What  does  N  N  represent?  What 
does  O  O  represent  ?    What  is  said  of  the  governor  ? 


148 


N  A  T  I  '  K  A  L  r  I  i  i  L  O  .<  O  P 11 Y . 


sliops,  factories,  <kc.  ;  and  different  directions  and  velocities 
may  be  given  to  the  motion  produced  by  the  miction  of  the 
steam  on  the  piston,  by  connecting  the  piston  to  the  beam  with 
wheels,  axles,  and  levers,  according  to  the  principles  stated 
under  the  head  of  Mechanics. 

191.  The  locomotive  engine  is  a  high-pressure  steam- 
engine,  mounted  on  wheels,  and  used  to  draw  loads 
on  a  railroad,  or  other  level  roads.  It  is  usually  accom- 
panied by  a  large  wagon,  called  a  tender,  in  which  the 
wood  and  water,  used  by  the  engine,  are  carried. 

Fig.  95  represents  a  side  view  of  the  internal  construction 
of  a  locomotive  steam-engine ;  in  which  F  represents  the  lire- 
box,  or  plac€  where  the  fire  is  kept ;  D  the  door  through  which 
the  fuel  is  introduced.  The  spaces  marked  B  are  the  inteiior 
of  the  boiler,  in  which  the  water  stands  at  the  height  indicated 
by  the  dotted  line.  The  boiler  is  closed  on  all  sides,  all  its 
openings  being  guarded  by  valves.  The  tubes  marked  j)  jj 
conduct  the  smoke  and  flame  of  the  fuel  through  the  boiler  to 
the  chimney  C  C,  serving,  at  the  same  time,  to  communicate 
T.he  heat  to  the  remotest  part  of  the  boiler.  By  this  arrange- 
ment none  of  the  heat  is  lost,  as  these  ttibes  are  all  stuTOund- 
ed  by  the  water.  S  S  S  is  the  steam-pipe,  open  at  the  top 
Y  S,  having  a  steam-tight  cock,  or  regulator,  Y,  which  is 
opened  and  shut  by  the  lever  L,  extending  outside  of  the 
boiler,  and  managed  by  the  engineer. 

The  operation  of  the  machine  is  as  follows :  the  steam  being 
generated  in  great  abundance  in  the  boiler,  and  being  unable 
to  escape  out  of  it,  acquires  a  considerable  degree  of  elastic 
force.  If  at  that  momen:  tlie  valve  Y  be  opened,  by  the 
handle  L,  the  steam  entering  the  pipe  S  passes  in  the  direction 
of  the  arrow,  through  the  tube,  and  enters  the  valve-box  at 
X.  There  a  shding- valve,  which  moves  at  the  same  time  with 
the  machine,  opens  for  the  steam  a  communication  successively 
with  each  end  of  the  cylinder  below.  Thus,  in  the  figure,  the 
entrance  on  the  right  hand  of  the  sliding-valve  is  represented 
as  being  open,  and  the  steam  follows  in  the  direction  of  the 
arrows  into  the  cylinder,  where  its  expansive  force  will  move 


191.  Describe  the  locomotive  steam-engine.  In  Fig.  95,  what  do  F 
and  D  represent  ?  What  do  the  following  references  respectively  repre- 
sent, namely,  S  S  S  ?  BBB?  pppp.^  CCl  X?  L?  P?  NN? 
RGKK? 


PYRONOMICS. 


149 


150 


NATURAL  PHILOSOPHY. 


the  piston  P  in  the  direction  of  the  arrow.  The  steam  or  air 
on  the  other  side  of  the  piston  passes  out  in  the  opposite 
direction,  and  is  convej^ed,  by  a  tube  passing  through  C  C, 
into  the  open  air. 

The  motion  of  the  piston  in  the  direction  of  the  arrow, 
causes  the  levers  N  N  to  close  the  slidinof-valve  on  the  riorht, 
and  open  a  communication  for  the  steam  on  the  opposite  side 
of  the  piston  ^,  where  it  drives  the  piston  back  towards  the 
arrow,  at  the  same  time  affording  a  passage  for  the  steam  on 
the  right  of  the  piston  to  pass  into  the  open  air. 

Motion  being  thus  given  to  the  piston,  it  is  communicated,  by 
means  of  the  rod  R  and  the  beam  G,  to  the  cranks  K  K,  which, 
being  connected  with  the  axle  of  the  wheel,  causes  it  to  turn, 
and  thus  move  the  machine. 

Thus  constructed,  and  placed  on  a  railroad,  the  locomotive 
steam-engine  is  advantageously  used  as  a  substitute  for  horse 
power,  for  drawing  heavy  loads.* 


THE  STATIONARY  STEAM-ENGINE. 

This  engine  is  generally  a  high-pressure,  or  non-condensing 
engine,  used  to  propel  machinery  in  workshops  and  factories. 
As  it  is  designed  for  a  labor-saving  machine,  it  is  desirable  to 
combine  simphcity  and  economy  with  safety  and  durability  in 
its  construction  ;  and  that  form  of  this  engine  is  to  be  preferred 
which  in  the  greatest  degree  unites  these  quahties.  The  figure 
on  page  151  represents  Tuft's  stationary  steam-engine,f  with  sec- 

*  The  apparatus  of  safety-valves  and  other  appliances  for  the  manage- 
ment of  the  power  produced  by  the  machine,  are  the  same  in  principle, 
though  differing  in  form,  with  those  used  in  other  steam-engines ;  for  a 
particular  description  of  which,  the  student  is  referred  to  practical  treatises 
upon  the  subject. 

t  This  engine  was  constructed  by  Mr.  Otis  Tufts  of  East  Boston,  Mas- 
sachusetts. It  is  the  engine  used  to  propel  the  machinery  at  the  late  Fair 
of  the  Massachusetts  Mechanic  Association,  where  it  was  very  highly  and 
justly  commended  for  its  beauty  and  simplicity  of  construction,  and  the 
perfectly  "  noiseless  tenor  of  its  way:'  The  figure  which  represents  it 
is  an  electrotype  copy  of  a  steel  plate,  designed  by  Brown  &  Harbrys. 
under  the  direction  of  Mr.  Tufts.  The  electrotype  copy  was  taken  by 
Mr.  A.  Wilcox,  Washington-street,  Boston.  The  electrotype  process  will 
bo  noticed  in  a  subsequent  page  of  this  volume. 


152 


NATURAL  PHILOSOPHY. 


tions  of  the  interior.  Like  the  double-acting  condensing  engine 
of  Mr.  Watt,  described  in  Fig.  94,  it  is  fumished  with  a  governor, 
by  which  the  supply  of  steam  is  regulated  ;  and  like  the  loco- 
motive, Fig.  95,  the  cylinder  with  its  piston  has  a  horizontal  posi- 
tion. The  steam  is  admitted  into  the  valve-box  thi'ough  an  aper- 
ture at  E,  in  the  section,  and  from  thence  passes  into  the  cylinder 
through  a  shding-valve,  alternately  to  each  side  of  the  piston  P, 
as  is  represented  by  the  direction  of  the  arrows,  the  sliding-valve 
being  moved  by  the  rod  V,  communicating  with  an  "  eccentric" 
apparatus  attached  to  the  axis  of  the  fjy-wheel.  The  direction 
of  the  current  of  steam  to  the  valve-box  is  represented  by  the 
arrow  at  I,  and  its  passage  outT\^ard  from  the  cylinder,  after  it 
has  moved  the  piston,  is  seen  at  0.  In  this  engine  there  is  no 
working-beam  as"  in  Watt's  engine,  Fig.  94,  but  the  motion  is 
communicated  from  the  piston-rod  to  a  crank  connected  with 
the  fly-wheel,  which,  turning  the  wheel,  will  move  all  ma- 
chinery connected  either  with  the  axis  or  the  circumference  of 
that  wheel. 


CHAPTER  X. 

OPTICS. 

192.  Optics  is  the  science  which  treats  of  light,  of 
colors,  and  of  vision. 

1.  The  science  of  optics  divides  all  substances  into  the 
following  classes ;  namely,  luminous,  transparent,  and  translu- 
cent ;  retlecting,  refracting,  and  opaque. 

2.  Luminous  bodies  are  those  which  shine  by  their  own  light ; 
such  as  the  sun,  the  stars,  a  burning  lamp,  or  a  fire.  ^ 

3.  Transparent  substances  are  those  which  allow  hght  to 
pass  through  them  freely,  so  that  objects  can  be  distinctly 
seen  through  them ;  as  glass,  water,  air,  &c. 


192.  Of  what  does  Optics  treat?  luto  what  classes  does  the  science  of 
Optics  divide  all  substances?  What  are  luminous  bodies?  Give  an  ex- 
ample of  a  luminous  body.    What  are  transparent  bodies  ? 


OPTICS. 


4.  Tr.inslucent  bodies  are  those  which  permit  a  portion  of 
light  to  pciss  through  them  ;  but  render  the  object  beliind 
them  indistinct ;  as  liorn,  oiled  paper,  colored  glass,  &c. 

5.  Reflecting  substances  are  those  which  do  not  permit  light 
to  pass  through  them ;  but  throw  it  off  in  a  direction  more  or 
less  oblique,  according  as  it  falls  on  the  reflecting  surface  ;  as 
polished  steel,  looking-glasses,  pohshed  metal,  &c. 

6.  Refracting  substances  are  those  which  turn  the  liglit 
from  its  course,  in  its  passage  through  them ;  and  opaque  sub- 
stances are  those  which  permit  no  light  to  pass  through  them ; 
as  metals,  wood,  (fee. 

7.  It  is  not  known  what  light  is.  Sir  Isaac  Newton  sup- 
posed it  to  consist  of  exceedingly  small  particles,  moving  from 
luminous  bodies ;  others  think  that  it  consists  of  the  undula- 
tions of  an  elastic  medium,  which  fills  all  space.  These  undu- 
lations (as  is  supposed)  produce  the  sensation  of  light  to  the 
eye,  in  the  same  manner  as  the  vibrations  of  the  air  produce 
the  sensation  of  sound  to  the  ear.  The  opinions  of  philosophers 
at  the  present  day  are  inclining  to  the  undulatory  theory. 

193.  A  ray  of  light  is  a  single  line  of  light  proceeding 
from  a  luminous  body. 

194.  Rays  of  light  are  said  to  diverge  when  they 
separate  more  widely,  as  they  pro-            Fig.  96. 
ceed  from  a  luminous  body.  

Fig.  96  represents  the  rays  of  light  di-  "^^^^^^^ 
verging  as  they  proceed  from  the  lumi-    ^  ^ 
nous  body,  from  F  to  D. 

195.  Rays  of  light  are  called  converging  when  tliey 
approach  each  other.    The  point  at 
which  converging  rays  meet  is  called 
the  focus. 

Fig.  97  represents  converging  rays  of 
light^  of  which  the  point  F  is  the  focus. 

Give  an  example  of  a  transparent  body.  What  are  translucent  bodies? 
Give  an  example  of  a  translucent  body.  What  are  reflecting  substances? 
Give  an  example  of  a  reflecting  body.  What  are  refracting  substances  ? 
What  are  opaque  substances?  What  is  light?  What  did  Sir  Isaac  New- 
ton suppose  it  to  be  ?  What  other  opinions  have  been  formed  concerning  it  ? 

193.  What  is  a  ray  of  light? 

194.  When  are  rays  of  light  said  to  diverge?    What  does  Fig.  96  rep- 


Fig.  97, 


154 


NATURAL  PHILOSOPHY. 


196.  A  beam  of  light  consists 
of  many  rays  running  in  parallel 


Fig.  98. 


lines. 


Fig.  98  represents  a  beam  of  light. 


197.  A  pencil  of  rays  is  a 
collection  of  diverging  or  converging  rays. 

198.  A  medium  is  any  substance,  solid  or  fluid, 
through  which  light  can  pass  ;  as  water,  glass,  air,  &c. 

199.  The  rays  of  light  which  issue  from  terrestrial 
bodies,  continually  diverge,  until  they  meet  with  a  re- 
fracting substance  ;  but  the  rays  of  the  sun  diverge  so 
little,  on  account  of  the  immense  distance  of  that  lumi- 
nary, that  they  are  considered  parallel. 

200.  Light,  proceeding  from  a  luminous  body,  is  pro- 
jected forward  in  straight  hues  in  every  possible  di- 
rection. It  moves  with  a  rapidity  but  little  short  of 
200,000  miles  in  a  second  of  time. 

201.  Every  point  of  a  luminous  body  is  a  centre, 
from  which  light  radiates  in  every  direction.  Rays 
of  light  proceeding  from  different  bodies,  cross  each 
other  without  interfering. 

202.  A  shadow  is  the  darkness  produced  by  the  in- 
tervention of  an  opaque  body,  which  prevents  the  rays 
of  light  from  reaching  an  object  behind  the  opaque 
body. 

Shadows  are  of  different  degrees  of  darkness,  because  the 
light  from  other  luminous  bodies  reaches  the  spot  where  the 


195.  When  are  rays  of  light  called  converging?  What  is  the  point,  at 
which  converging  rays  meet,  called  ? 

196.  What  is  a  beam  of  light?    What  does  Fig.  98  represent? 

197.  What  is  a  pencil  of  light? 

198.  What  is  a  medium? 

199.  In  what  manner  do  the  rays  of  light  proceed  from  terrestrial  bodies  ? 
In  what  kind  of  lines  do  the  rays  of  light  proceed  from  the  san  ? 

200.  In  what  way  ifs  light  projected  forward  from  any  luminous  body? 
With  what  rapidity  does  it  move  ? 

201.  From  what  point,  in  a  luminous  body,  does  light  radiate? 

202.  How  is  a  shadow  produced  ?  Why  are  shadows  of  different  de- 
grees of  darkness  ? 


OPTICS. 


155 


shadow  is  formed.  Thus,  if  a  shadow  be  formed  when  two 
candles  are  burning  in  a  room,  that  sliadow  will  be  both 
deeper  and  darker  if  one  of  the  candles  be  extinguished.  The 
darkness  of  a  shadoAV  is  proportioned  to  the  intensity  of  the 
light,  when  the  shadow  is  produced  by  the  interruption  of  the 
rays  from  a  single  luminous  body.'^' 

203.  When  a  luminous  body  is  larger  than  an  opaque 
body,  the  shadow  of  the  opaque  body  will  gradually 
diminish  in  size  till  it  terminates  in  a  point.  The  form 
of  the  shadow  of  a  spherical  body 
will  be  that  of  a  cone.  a 


Fig.  99.  A  represents  the  sun,  and  ^ 
B  the  moon.    The  sun,  being  much 
larger  than  the  moon,  causes  it  to 
cast  a  converging  shadow,  which  terminates  at  E. 

204.  When  the  luminous  body  is  smaller 
opaque  body,  the  shadow 
of  the  opaque  body  grad- 
ually increases  in  size 
with  the  distance,  with- 
out limit. 


than  the 


Fig.  100. 


In  Fig.  100  the  shadow 
of  the  object.  A,  increases 
in  size  at  the  different  dis- 
tances, B,  C,  D,  E,  or,  in  other  words,  it  constantly  diverges. 

205.  When  several  luminous  bodies  shine  upon  the 

*  As  the  degree  of  light  and  darkness  can  be  estimated  only  by  com- 
parison, the  strongest  light  will  appear  to  produce  the  deepest  shadow. 
Hence,  a  total  eclipse  of  the  snn  occasions  a  more  sensible  darkness  than 
midnight,  because  it  is  immediately  contrasted  with  the  strong  light  of 
dav. 


To  what  is  the  darkness  of  a  shadow  proportioned,  when  the  shadow  is 
produced  by  the  interruption  of  the  rays  from  a  single  luminous  body  ? 

203.  What  is  said  of  the  shadow  of  the  oi)aque  body  when  the  luminous 
body  is  the  larger  ?    Explain  Fig.  99. 

204.  What  is  said  of  the  shadow  of  the  opaque  body  when  the  luminous 
body  is  the  smaller  ?    Explain  Fig.  100. 

205.  How  many  shadows  are  produced  when  several  luminous  bodies 
shine  upon  the  same  object? 


NATURAL  PHILOSOPHY. 


156 

same  object,  each  one  will 
produce  a  shadow. 

Fig.  101  represents  a  ball, 
A,  illuminated  by  the  three 
candles,  B,  C,  and  D.  The 
light  B  produces  the  shadow 
h,  the  light  C,  the  shadow  c, 
and  the  light  D,  the  shadow 
d  ;  but  as  the  light  from  each 
of  the  candles  shines  upon  all 
the  shadows,  except  its  own, 
the  shadows  will  be  faint. 


Fig.  lOL 


206.  When  rays  of  light  fall  upon  an  opaque  body, 
.part  of  them  are  absorbed,  and  part  are  reflected. 

Light  is  said  to  be  reflected  when  it  is  thrown  off  from  the 
body  on  which  it  falls ;  and  it  is  reflected  in  the  largest  quan- 
tities from  the  most  highly  polished  suifaces.  Thus,  although 
most  substances  reflect  it  in  a  degree,  polished  metals,  looking- 
glasses,  or  mirrors,  &c.,  reflect  it  in  so  perfect  a  manner  as  to 
convey  to  our  eyes,  when  situated  in  a  proper  position  to  re- 
ceive them,  perfect  images  of  whatever  objects  shine  on  them, 
either  by  their  own,  or  by  borrowed  light. 

207.  That  part  of  the  science  of  Optics  which  relates 
to  reflected  light,  is  called  Catoptrics. 

208.  The  laws  of  reflected  light  are  the  same  as  those 
of  reflected  motion.  Thus,  when  light  falls  perpen- 
dicularly on  an  opaque  body,  it  is  reflected  back  in  the 
same  line,  towards  the  point  whence  it  proceeded.  •  If 
it  fall  obliquely,  it  will  be  reflected  obliquely  in  the  op- 
posite direction  ;  and  in  all  cases  the  angle  of  incidence 


Explain  Fig.  101. 

206.  What  is  the  consequence  when  rays  of  light  fall  upon  an  opaque 
body  which  they  cannot  pass  ?    What  is  meant  by  the  reflection  of  light  ? 

207.  What  are  Catoptrics  ? 

208.  By  what  laws  is  light  governed  ?  How  is  light  reflected  when  it 
falls  perpendicularly  on  an  opaque  body  ?  How  is  it  reflected  when  it  falls 
obliquely  ?  How^  do  the  angles  of  incidence  and  reflection  compare  with 
each  other?  By  what  light  are  opaque  objects  seen?  By  what  light  are 
luminous  objects  seen  ? 


OPTICS. 


157 


\vill  ho  (M|ual  to  the  anu^lc  ol'  rcllection.  Tliis  is  1,li(3 
i'undaiuontal  la  w  of  Catoptrics,  or  reflected  li,^ht.* 

()[)n(|iic  objocts  are  scon  only  by  rcilecJed  liij^lit.  Jjuniinoiis 
bodies  are  seen  by  the  rays  of  light  which  tlu^y  send  directly 
to  our  eyes. 

209,  All  bodies  absorb  a  portion  of  the  lii^dit  whicii 
they  receive  ;  therefore  the  intensity  of  light  is  diminish- 
ed every  time  that  it  is  reflected. 

*  The  angles  of  incidence  and  reflection  have  already  been  described 
under  tiie  head  of  Mechanics;  but  as  all  the  phenomena  of  reflected  li<^ht 
depend  upon  the  law  stated  above,  and  a  clear  idea  of  those  angles  is  ne- 
cessary in  order  to  understand  the  law,  it  is  deemed  expedient  to  repeat 
in  this  connexion  the  explanation  already  given. 

An  incident  ray  is  a  ray  proceeding  to^  or  falling  on  any  surface  ;  and 
a  reflected  ray  is  the  ray  which  proceeds  from  any  reflecting  surface. 

Fig.  10*2  is  designed  to  show  the  angles  of  inci- 
dence and  of  reflection.  In  this  figure  M  A  M  is  a  102. 
mirror,  or  reflecting  surface.  P  is  a  line  perpendic- 
ular to  the  surface.  I A  represents  an  incident  ray, 
falling  on  the  mirror  in  such  a  manner  as  to  form, 
with  the  perpendicular  P,  the  angle  I  A  P.  This  is 
called  the  angle  of  incidence.  The  line  R  A  is  to 
be  drawn  on  the  other  side  of  P  A  in  such  a  manner 
as  to  have  the  same  inclination  with  P  A  as  I  A 
has :  that  is,  tiie  angle  R.  A  P  is  equal  to  I  A  P. 
The  line  R  A  will  then  show  the  coarse  of  the  re- 
flected ray ;  and  the  angle  RAP  will  be  the  angle  of  reflection. 

From  whatever  surface  a  ray  of  light  is  reflected,  whether  it  be  a  plain 
surface,  a  convex  surface,  or  a  concave  surface,  this  law  invariably  pre- 
vails; so  that  if  we  notice  the  inclination  of  any  incident  ray,  and  the 
situation  of  the  perpendicular  to  the  surface  on  which  it  falls,  we  can  al- 
ways determine  in  what  manner,  or  to  what  point  it  will  be  reflected. 
This  law  explains  the  reason  why,  when  we  are  standing  on  one  side  of  a 
mirror,  we  can  see  the  reflection  of  objects  on  the  opposite  side  of  the 
room,  but  not  those  on  the  same  side  on  which  we  are  standing.  It  also 
explains  the  reason  why  a  person  can  see  his  whole  figure  in  a  mirror  not 
more  than  half  of  his  height.  It  also  accounts  for  all  the  apparent  pecu- 
liarities of  the  reflection  of  the  different  kinds  of  mirrors. 


Explain  the  angles  of  incidence  and  reflection.    Is  the  same  principle 
applicable  to  all  kinds  of  surfaces  ?    Explain  its  application  to  min  ors. 
209.  Why  is  the  intensity  of  light  diminished  every  time  it  is  reflected  ? 


158 


NATURAL  THILOSOPHY. 


210.  Every  portion  of  a  reflecting  surface  reflects  an 
entire  image  of  the  luminous  body  shining  upon  it. 

1.  When  the  sun  or  the  moon  shines  upon  a  sheet  of  water, 
every  portion  of  the  surface  reflects  an  entire  image  of  the  lu- 
minary ;  but  as  the  image  can  be  seen  only  by  reflected  rays, 
and  as  the  angle  of  reflection  is  always  equal  to  the  angle  of 
incidence,  the  image  for  any  point  can  be  seen  only  in  the  re- 
flected ray  prolonged. 

2.  Objects  seen  by  moonlight  appear  fainter  than  when  seen 
by  daylight,  because  the  light  by  which  they  are  seen  has  been 
twice  reflected;  for,  the  moon  is  not  a  luminous  body,  but  its 
light  is  caused  by  the  sun  shining  upon  it.  This  light,  reflect- 
ed from  the  moon  and  falling  upon  any  object  is  again  reflect- 
ed by  that  object.  It  suffers,  therefore,  two  reflections ;  and 
since  a  portion  is  absorbed  by  each  surface  that  reflects  it,  the 
light  must  be  proportionally  fainter.  In  traversing  the  atmo- 
sphere, also,  the  rays,  both  of  the  sun  and  moon,  suffer  diminu- 
tion ;  for,  although  pure  air  is  a  transparent  medium,  which 
transmits  the  rays  of  light  freely,  it  is  generally  surcharged 
with  vapors  and  exhalations,  by  which  some  portion  of  light 
is  absorbed. 

211.  All  objects  are  seen  by  means  of  the  rays  of 
light  .emanating  or  reflected  from  them  ;  and  therefore 
when  no  light  falls  upon  an  opaque  body  it  is  invisible. 

This  is  the  reason  why  none  but  luminous  bodies  can  be 
seen  in  the  dark.  For  the  same  reason,  objects  in  the  shade, 
or  in  a  darkened  room  appear  indistinct,  while  those  which  are 
exposed  to  a  strong  light  can  be  clearly  seen. 

212.  When  rays  of  light,  proceeding  from  any  object, 
enter  a  small  aperture,  they  cross  one  another  and  form 
an  inverted  image  of  the  object.* 

*  This  is  a  necessary  consequence  of  the  law  that  light  always  moves 
in  straight  lines.  /^ 

210.  Does  every  portion  of  a  reflecting  surface  reflect  an  entire  image 
cf  the  luminous  body  shining  upon  it?  When  the  sun  or  moon  shines  upon 
a  sheet  of  water,  why  do  we  not  see  an  image  reflected  from  every  portion 
of  the  surface  ?  Why  do  objects,  seen  by  moonlight,  appear  fainter  than 
when  seen  by  dayhght?  By  what  hght  does  the  moon  shine  ?  What  ab- 
sorbs some  of  the  rays  of  light  in  traversing  the  atmosphere  ? 

211.  How  are  all  objects  seen?  Why  can  none  but  luminous  bodies  be 
seen  in  the  dark  ? 


or  ric; 


Fii^.  lO;^  rej)i(>s(Mits  the  lavs  ftoin  an  ()])j('C'l,  ^/ r,  ctilci-iiifr 
an  aj)iMtui-(\     'Vhc  v:i\  iVoin  (f  passes  down 
throuj^"h  the  ajx'ilure  to  (/,  and  the  ra\'  iVoin  j.,^,,  k,;^. 

('  passos  up  to  i),  aiul  thus  these  rays,  cioss- 
ing-  at  tlio  apertuiv,  I'oiin  an  inverted  iinai^(; 
on  the  wall.  The  room  in  which  this  vx- 
j)oriniont  is  made  should  be  darkened,  and 
no  lio-ht  permitted  to  enter,  excepting  through 
the  aperture.  It  then  becomes  a  camera 
obscura."^ 

213.  The  angle  of  vision  is  the  angle  formed  at  the 
eye  by  two  lines  drawn  from  opposite  parts  of  an  ob- 
ject. 

1.  The  angle  C,  in  ^^s-  104.  a 
Fig.  104,  represents 
the  angle  of  vision. 
The  line  A  C  proceed- 
ing from  one  extremi- 
ty of  the  object  meets 

the  line  B  C  proceeding  b 
from  the  opposite  ex- 
tremity, and  forms  an  angle  C  at  the  eye; — this  is  the  angle 
of  vision. 

2.  Fig.  105  represents  the  different  angles,  made  by  the 
same  object,  at  different  distances.    From  an  inspection  of  the 

*  These  words  signify  a  darkened  chamber.  In  the  future  description 
which  will  be  given  of  the  eye,  it  will  be  seen  that  the  camera  obscura  is 
constructed  on  the  same  principle  as  the  eye.  If  a  convex  lens  be  placed 
in  the  aperture,  an  inverted  picture,  not  only  of  a  single  object,  but  of  the 
entire  landscape,  will  be  found  on  the  wall.  A  portable  camera  obscura  is 
made  by  admitting  the  light,  into  a  box  of  any  size,  through  a  convex 
lens,  which  throws  the  image  upon  an  inclined  mirror,  from  whence  it  is 
reflected  upwards  to  a  plate  of  ground  glass.  In  this  manner  a  beautiful 
but  diminished  image  of  the  landscape,  or  of  any  group  of  objects,  is  pre- 
sented on  the  plate  in  an  erect  position. 


212.  Wbat  kind  of  an  image  is  formed  when  rays  of  light,  proceeding 
from  an  obiect,  enter  a  small  aperture'?  Illustrate  this  by  Fig.  103.  Vv^hat 
is  a  camera  obscura?    How  can  a  portable  camera  obscura  be  made? 

213.  How  is  the  angle  of  vision  formed?  Explain  Fig.  104.  What 
does  Fig.  105  represent? 


lf>0 


NATURAL  PHILOSOPHY. 


figure  it  is  e\i(ient,  that  the  nearer  an 
object  is  to  the  eye,  the  wider  must 
be  the  opening  of  the  hues  to  admit 
the  extremities  of  the  object;  and, 
consequently,  the  larger  the  angle 
under  which  it  is  seen ;  and,  on  the 
contrary,  that  objects  at  a  distance 
will  form  small  angles  of  vision.  Thus, 
in  this  figui-e,  the  three  crosses,  F  G,  D  E,  and  A  B,  are  all  of 
the  same  size;  but  A  B,  being  the  most  distant,  subtends  the 
smallest  angle'^  A  C  B,  while  D  E  and  F  G,  being  nearer  to  the 
eye,  situated  at  C,  form  respectively  the  larger  angles,  D  C  E 
and  F  C  G. 

214.  When  an  object,  at  any  distance,  does  not  sub- 
tend an  angle  of  more  than  two  seconds  of  a  degree,  it 
is  invisible. 

At  the  distance  of  four  miles  a  man  of  common  stature  will 
thus  become  imisible,  because  his  height  at  that  distance  will 
not  subtend  an  angle  of  two  seconds  of  a  degree.    The  size  of 

*  The  apparent  size  of  an  object  depends  upon  the  size  of  the  angle  of 
vision.  But  we  are  accustomed  to  correct,  by  experience,  the  fallacy  of 
appearances  ;  and,  therefore,  since  we  know  that  real  objects  do  not  vary 
in  size,  but  that  the  angles  under  which  we  see  thera  do  vary  with  the 
distance,  we  are  not  deceived  by  the  variations  in  the  appearance  of  ob-  . 
jects.  Thus,  a  house  at  a  distance  appears  absolutely  snaaller  than  the 
window  through  which  we  look  at  it ;  otherwise  w^e  could  not  see  it 
through  the  window  ;  but  our  knowledge  of  the  real  size  of  the  house  pre- 
vents our  alluding  to  its  apparent  magnitude.  In  Fig.  104  it  will  be  seen 
that  the  several  crosses,  AB,  DE,  FG,  and  HI,  although  very  different  in 
size,  on  account  of  their  different  distances,  subtend  the  same  angle  AC  B  ; 
they,  therefore,  all  appear  to  the  eye  to  be  of  the  same  size,  while,  in  Fig. 
105,  the  three  objects  AB,  DE,  and  FG,  although  of  the  same  absolute 
size,  are  seen  at  a  different  angle  of  vision,  and  they,  therefore,  will  seem 
of  different  sizes,  appearing  larger,  as  they  approach  the  eye. 

It  is  upon  a  correct  observance  of  the  angle  of  vision  that  the  art  of  per- 
spective drawing  is  indebted  for  its  accuracy. 

What  effect  has  the  nearness  of  the  object  to  the  eye,  on  the  angle? 
Illustrate  this  by  the  figure.  Upon  what  does  the  apparent  size  of  an  ob- 
ject depend  ?  Why  do  objects  appear  so  large?  To  what  is  the  ari:  Oi 
perspective  drawing  indebted  for  its  accuracy  ? 

214.  How  large  an  angle  must  a  body  subtend  to  be  visible? 


OPTICS. 


1(31 


tlie  apparent  diameter  of  the  heavenly  bodies  is  generally 
stated  by  the  angle  which  they  subtend. 

215.  When  the  velocity  of  a  moving  body  does  not 
exceed  tw^enty  degrees  in  an  hour,  its  motion  is  imper- 
ceptible to  the  eye. 

1.  It  is  for  this  reason  that  the  motion  of  the  heavenly  bodies 
is  invisible,  notwithstanding  their  immense  velocity. 

2.  The  real  velocity  of  a  body  in  motion  ro  nd  a  point,  de- 
pends on  the  space  comprehended  in  a  degree.  The  more 
distant  the  moving  body  from  the  centre, 

or,  in  other  words,  the  larger  the  circle  ^^^^ 
which  it  has  to  describe,  the  larger  will  be  $ 
the  deoTce.  -^.A 

3.  In  Fig.  106,  if  the  man  at  A,  and  the 
man  at  B,  both  start  together,  it  is  manifest 
that  A  must  move  more  rapidly  than  B,  to  / 
arrive  at  C  at  the  same  time  that  B  reaches 
D  ;  because  the  arc  AC  is  the  arc  of  a     o       »      |^  e 
larger  circle  than  the  arc  B  D.    But  to 

the  eye  at  E,  the  velocity  of  both  appears  to  be  the  same,  be- 
cause both  are  seen  under  the  same  angle  of  vision. 

216.  There  are  three  kinds  of  mirrors,^  namel}%  the 
plain,  the  concave,  and  the  convex  mirror. 

Plain  mirrors  are  those  which  have  a  flat  surface, 
such  as  a  common  looking-glass ;   and  they  neither 

*  A  mirror  is  a  smooth  and  polished  surface,  that  forms  images  by  tlio 
reflection  of  the  rays  of  hght.  Mirrors  (or  looking-glasses)  are  made  of 
glass,  with  the  back  covered  with  an  amalgam,  or  mixtm'e  of  mercury  and 
tinfoil.  It  is  the  smooth  and  bright  surface  of  the  mercury  that  reflects 
the  rays,  the  glass  acting  only  as  a  transparent  case,  or  covering,  through 
which  the  rays  find  an  easy  passage.  Some  of  the  rays  are  absorbed  in 
their  passage  through  the  glass,  because  the  purest  glass  is  not  free  from 
imperfections.  For  this  reason,  the  best  mirrors  are  made  of  fine  and 
highly-polished  steel. 


215.  When  is  the  motion  of  a  body  invisible  ?  Why  is  the  motion  of 
the  heavenly  bodies  invisible  ?  Upon  what  does  the  real  velocity  of  a 
body,  in  motion  round  a  point,  depend?  Explain  Fig.  106.  Why  does 
the  velocity  of  both,  to  an  eye  at  E,  appear  to  be  the  same  ? 

1:216.  How  many  kinds  of  mirrors  are  there?  What  are  plain  mirrors? 
Do  tiiey  magnify  or  diminish  the  object? 


162 


NATUR ATi  PIIILOSDriiY. 


magnify  nor  diminish  the  image  of  objects  reflected 
from  them. 

A  convex  mirror  is  a  portion  of  the  external  surface 
of  a  sphere.  Convex  mirrors  have  therefore  a  convex 
surface. 

A  concave  mirror  is  a  portion  of  the  inner  surface  oi 
a  hollow  sphere.  Concave  mirrors  have  therefore  a 
concave  surface. 

In  Fig.  107,  MIS'  lepresents  both  a  convex  and  a  concave 
mirror.  They  are  both  a  portion  of  a  sphere  of  which  0  is  the 
centre.     The  outer  part 

of  M  N  is  a  convex,  and  g 
the  inner  part  is  a  concave 
mirror.  Let  AB,  CD,  EF, 
represent  rays  falling  on  / 
the  convex  mirror  Si  N.  / 
As  the  three  rays  are  ( 
parallel,  they  would  all  be  \ 
perpendicular  to  a  plane 
or  llat  mirror ;  but  no  ray 
cam,  fall  peiyendicularJy  H 
on  a  concave  or  convex  mir- 
ror, ichich  is  not  directed  towards  the  centre  of  the  sphere  of  ivhich 
the  mirror  is  a  portion.  For  this  reason  the  ray  C  D  is  perpen- 
dicular to  the  mirror ;  while  the  other  rays  A  B  and  E  F  fall, 
obliquely  upon  it.  The  middle  ray  therefore  falling  perpendicu- 
larly on  the  mirror,  will  be  reflected  back  in  the  same  line, 
while  the  two  other  rays  falling  obliquely  will  be  reflected 
obliquely ;  namely,  the  ray  A  B  will  be  reflected  to  G,  and  the 
ray  E  F  to  H,  and  the  angles  of  incidence  A  B  P  and  EFT 
will  be  equal  to  the  angles  of  reflection  P  B  G  and  T  F  H,  and 
since  we  see  objects  in  the  direction  of  the  reflected  rays,  we  shall 
see  the  image  at  L,  which  is  the  point  at  which  the  reflected 
rays  if  continued  through  the  mirror  would  unite  and  form  the 
image.  This  point  is  equally  distant  from  the  surface,  and  the 
centre  of  the  sphere,  and  is  called  the  imaginary  focus  of  the 
mirror.     It  is  called  the  imaginary  focus,  because  the  rays  do 


.  A 

 j  • 

 S—  E 

What  are  convex  mirrors?  What  part  of  a  sphere  is  a  convex  mir- 
ror? What  are  concave  mirrors?  What  part  of  a  sphere  is  a  concave 
mirror?  In  Fig.  107,  which  part  of  the  sphere  represents  a  convex  mir- 
iror?    Which  part  a  concave  mirror?    Explain  the  %nre. 


OPTICS. 


163 


not  really  unite  at  tliat  point,  but  only  appear  to  do  so ;  for 
the  rays  do  not  pass  tlirougli  the  mirroi-,  since  they  are  reflect- 
ed by  it. 

217.  The  image  of  an  object  reflected  from  a  convex 
mirror  is  smaller  than  the  object. 

This  is  owing  to  the  divergence  of  the  reflected  rays.  A 
convex  mirror  converts,  hj  reflection,  parallel  rays  into  divergent 
rays ;    rays  that 

fall  upon  the  mir-  Fig.  108, 

ror  divergent,  are  ^ 
rendered  still  more 
divergent  by  re- 
flection, and  con- 
vergent rays  are 
reflected  either 
parallel,  or  less 
convergent.  If, 
then,  an  object, 
A  B,  be  placed 
before  any  part 
of  a  convex  mir- 
ror, the  two  rays  A  and  B  proceeding  from  the  extremities, 
falling  convergent  on  the  mirror,  will  be  reflected  less  con- 
vergent, and  will  not  come  to  a  focus  until  they  arrive  at  C  ; 
then  an  eye  placed  in  the  direction  of  the  reflected  rays  will 
see  the  image  formed  in  (or  rather  behind)  the  mirror  at  a  h  ; 
and  as  the  image  is  seen  under  a  smaller  angle  than  the  object, 
it  will  appear  smaller  than  the  object. 

218.  The  true  focus  of  a  concave  mirror  is  a  point 
equally  distant  from  the  centre  and  the  surface  of  the 
sphere,  of  which  the  mirror  is  a  portion. 

219.  When  an  object  is  further  from  the  concave 
mirror  than  its  focus,  the  image  will  be  inverted  ;  but 
when  the  object  is  between  the  mirror  and  its  focus, 


217.  How  does  the  image  of  an  object  reflected  from  a  convex  mirror 
compare  with  the  object?    Give  the  illustration. 

218.  What  is  the  focus  of  a  concave  mirror? 

219.  When  an  object  is  further  from  the  concave  mirror  than  the  focus, 
how  will  the  image  appear?  When  the  object  is  between  the  mirror  and 
the  focus  ? 


164 


NATURAL  PHILOSOPHY. 


the  image  will  be  upright,  and  grow  larger  in  proportion 
as  the  object  is  placed  nearer  to  the  mirror.* 

220.  The  image  reflected  by  a  concave  mirror  is  lar- 
ger than  the  object,  when  the  object  is  placed  between 
the  mirror  and  its  focus. f 

1.  This  is  owing  to  the  convergent  property  of  the  concave 
mirror.    If  the  object  A  B  be  placed  between  the  concave 
mirror  and  its  fo- 
cus /,  the  rays  A  Fig-  109. 
and  B  from  its  ex- 
tremities will  fall 
divergent  on  the 
mirror,    and,  on 
being  reflected, 
become   less  di- 
vergent, as  if  they 
proceeded  from 
C.     To  an  eye 
placed  in  that  sit- 
uation, namely,  at 

*  Concave  mirrors  have  the  peculiar  property  of  forming  images  in  the 
air.  The  mirror  and  the  object  being  concealed  behind  a  screen,  or  a  wall, 
and  the  object  being  strongly  illuminated,  the  rays  from  the  object  fail 
upon  the  mirror,  and  are  reflected  by  it  through  an  opening  in  the  screen 
or  wall,  forming  an  image  in  the  air.  Showmen  have  availed  themselves 
of  this  property  of  concave  mirrors,  in  producing  the  appearance  of  appa- 
ritions, which  have  terrified  the  young  and  the  ignorant.  These  images 
have  been  presented  with  great  distinctness  and  beauty,  by  raising  a  fine 
transparent  cloud  of  blue  smoke,  by  means  of  a  chafing-dish,  around  the 
focus  of  a  large  concave  mirror. 

t  There  are  three  cases  to  be  considered  with  regard  to  the  efl^ects  of 
concave  mirrors  : 

1.  When  the  object  is  placed  between  the  mirror  and  the  principal 
focus. 

2.  When  it  is  situated  between  its  centre  of  concavity  and  that  focus. 

3.  When  it  is  more  remote  than  the  centre  of  concavity. 

1st.  In  the  first  case,  the  rays  of  light  div^f^iiig  after  reflection,  but  in  a 


How  nmst  the  object  be  placed  that  the  image  may  appear  upright  ? 
And  as  the  object  is  removed  towards  the  mirror? 

220.  If  the  object  be  placed  between  the  mirror  and  the  focus,  how  does 
the  image  compare  with  the  object? 


OPTICS. 


1G5 


C,  the  image  will  appear  magnified  beliiad  the  min  or,  at  a  h, 
since  it  is  seen  under  a  larger  angle  than  the  object. 

2.  Tlie  following  facts  result  from  the  operation  of  the  law 
already  stated  as  the  fundamental  law  of  Catoptrics,  namely, 
that  the  angles  of  incidence  and  reflection  are  always  equal. 

3.  In  estimating  these  angles,  it  must  be  recollected,  that  no 
hne  is  perpendicular  to  a  convex  or  concave  mirror,  which  will 
not,  when  sufficiently  prolonged,  pass  through  the  centre  of  the 
sphere  of  which  the  mirror  is  a  portion. 

4.  The  truth  of  these  statements  may  be  illustrated  by  simple 
drawings  ;  always  recollecting,  in  drawing  the  figures,  to  make 
the  angles  of  incidence  and  reflection  equal.  The  whole  may 
also  be  shown  by  the  simple  experiment  of  placing  the  flame 
of  a  candle  in  various  positions,  before  both  convex  and  concave 
mirrors. 

FACTS  WITH  REGARD  TO  CONVEX  MIRRORS. 

1.  Parallel  rays  reflected  from  a  convex  surface,  are  made 
to  diverge. 

less  degree  than  before  such  reflection  took  place,  the  image  will  be  larger 
than  the  object,  and  appear  at  a  greater  or  smaller  distance  from  the  sur- 
face of  the  mirror,  and  behind  it.    The  image  in  this  case  will  be  erect. 

2d.  When  the  object  is  between  the  principal  focus  and  the  centre  of  the 
mirror,  the  apparent  image  will  be  in  front  of  the  mirror,  and  beyond  the 
centre,  appearing  very  distant  when  the  object  is  at  or  just  beyond  the 
focus,  and  advancing  towards  it  as  it  recedes  towards  the  centre  of  con- 
cavity, where,  as  already  stated,  the  image  and  the  object  will  coincide. 
During  the  retreat  of  the  object,  the  image  will  still  be  inverted,  because 
the  rays  belonging  to  each  visible  point  will  not  intersect  before  they  reach 
the  eye.  But  in  this  case,  the  image  becomes  less  and  less  distinct,  at  the 
same  time  that  the  visual  angle  is  increasing  ;  so  that  at  the  centre,  or 
rather  a  little  before,  the  image  becomes  confused  and  imperfect,  owing  to 
the  small  parts  of  the  object  subtending  angles  too  large  for  distinct  vision, 
just  as  happens  when  objects  are  viewed  too  near  with  the  naked  eye. 

3d.  In  the  cases  just  considered,  the  images  will  appear  erect ;  but  in  the 
case  where  the  object  is  further  from  the  mirror  than  its  centre  of  con- 
cavity, the  image  will  be  inverted.  The  more  distant  the  object  is  from 
the  centre,  the  less  will  be  its  image ;  but  the  image  and  object  will  coin- 
cide when  the  latter  is  stationed  exactly  at  the  centre. 

What  peculiar  properties  have  concave  mirrors?  What  facts  are  stated 
with  regard  to  convex  mirrors,  as  resulting  from  the  fundamental  law  of 
Catoptrics  1 


166 


NATURAL  PHILOSOPHY. 


2.  Diverging  rays  reflected  from  a  convex  surface,  are  made 
more  diver oino-. 

3.  When  converging  rays  tend  towards  the  focus  of  parallel 
rays,  they  will  become  parallel  when  reflected  from  a  convex 
surface. 

4.  When  converging  rays  tend  to  a  point  nearer  the  surface 
than  the  focus,  they  will  converge  less  when  reflected  from  a 
CONVEX  smface. 

5.  If  converging  rays  tend  to  a  point  between  the  focus  and 
the  centre,  they  will  diverge  as  from  a  point  on  the  other  side 
of  the  centre,  farther  from  it  than  the  point  towards  which 
they  converged. 

G.  If  converging  rays  tend  to  a  point  beyond  the  centre, 
they  will  diverge  as  from  a  point  on  the  contrary  side  of  the 
centre,  nearer  to  it  than  the  point  towards  which  they  con- 
verged. 

7.  If  converging  rays  tend  to  the  centre,  when  reflected,  they 
will  proceed  in  a  direction  as  far  from  the  centre. 


FACTS  WITH  REGARD  TO  COXCAVE  MIRRORS. 

1.  Parallel  rays,  reflected  from  a  concave  surface,  are  made 
converging. 

2.  Converging  rays,  falhng  upon  a  conca^-e  surface,  are 
made  to  converge  more. 

3.  Diverging  rays,  falhng  upon  a  concave  surface,  if  they 
diverge  from  the  focus  of  parallel  rays,  become  parallel. 

4.  If  from  a  point  nearer  to  the  sui'face  than  that  focus,  they 
diverge  less  than  before  reflection. 

o.  If  from  a  point  between  that  focus  and  the  centre,  they 


What  is  said  of  parallel  rays  ?  What  is  said  of  diverging  rays  ?  W^hat 
is  said  of  converging  rays,  when  they  tend  towards  the  focus  of  parallel 
rays  ?  What  is  said  of  converging  ra3^s,  when  they  tend  to  a  point  nearer 
the  surface  than  the  focus  1  What  is  said  of  converging  rays,  when  they 
tend  to  a  point  between  the  focus  and  the  centre?  What  is  said  of  con- 
verging rays,  when  they  tend  to  a  point  beyond  the  centre?  What  is 
said  of  converging  rays,  when  they  tend  to  the  centre? 

NVhat  is  said  with  regard  to  parallel  rays,  when  reflected  from  a  con- 
cave surface  ?  What  is  said  of  converging  rays  ?  What  is  said  of  di- 
verging rays,  if  they  diverge  from  a  focus  of  parallel  rays  ?  What,  if  from 
a  point  nearer  to  the  surface  than  that  focus  ? 


OPTICS. 


1G7 


converge,  after  reflection,  to  some  point  on  the  contrary  side  of 
the  centre,  and  fartlier  from  the  centre  than  the  point  from 
which  they  diverged. 

G.  If  from  a  point  beyond  the  centre,  the  reflected  rays  will 
convei-ge  to  a  point  on  the  contrary  side,  but  nearer  to  it  than 
the  point  from  which  they  diverged. 

7.  If  from  the  centre,  they  will  be  reflected  thither  again. 

REFRACTION  OF  LIGHT. 

221.  That  part  of  the  science  of  Optics  which  treats 
of  refracted  light  is  called  Dioptrics. 

222.  By  the  refractionf  of  light  is  meant  its  being 
turnt?d  or  bent  from  its  course  ;  and  this  always  takes 
place  when  it  passes  obliquely  from  one  medium  to 
another. 

223.  By  a  medium, J  in  Optics,  is  meant  any  substance 
through  which  light  can  pass.  Thus,  air,  glass,  water, 
and  other  fluids,  are  media. 

*  Concave  mirrors,  by  the  property  which  they  possess  of  causing  par- 
allel rays  to  converge  to  a  focus,  are  sometimes  used  as  burning-glasses. 
M.  Dufay  made  a  concave  mirror  of  plaster  of  Paris,  gilt  and  burnished, 
20  inches  in  diameter,  with  which  he  set  fire  to  tinder  at  the  distance  of 
50  feet.  But  the  most  remarkable  thing  of  the  kind  on  record,  is  the  com- 
pound mirror  constructed  by  Buffon.  He  arranged  168  small  plane  mirrors 
in  such  a  manner  as  to  reflect  radiant  light  and  heat  to  the  same  focus, 
like  one  large  concave  mirror.  With  this  apparatus,  he  was  able  to  set 
wood  on  fire  at  the  distance  of  209  feet,  to  melt  lead  at  100  feet,  and  silver 
at  50  feet. 

t  The  power  of  being  refracted  is  called  refrangihility. 

X  The  plural  number  of  this  word  is  media,  although  mediums  is  some- 
times used.  A  medium  is  called  dense  or  rare,  in  optics,  according  to  its 
refractive  power,  and  not  according  to  its  specific  gravity.  Thus,  alcohol, 
and  many  of  the  essential  oils,  although  of  less  specific  gravity  than  water. 


What,  if  from  a  point  between  that  focus  and  the  centre  ?  If  from  a 
point  beyond  the  centre?    If  from  the  centre  ? 

221.  What  is  Dioptrics? 

222.  What  is  meant  by  the  refraction  of  light?  When  does  this  take 
place  ? 

223.  What  is  a  medium,  in  Optics?  Give  some  examples  of  media. 
Note.  In  what  proportion  is  a  medium  dense  or  rare  ? 


168 


NATURAL  PHILOSOPHY. 


224.  There  are  three  fundamental  laws  of  Dioptrics, 
on  which  all  its  phenomena  depend,  namely : 

1st.  When  light  passes  from  one  medium  to  another,  in 
a  direction  perpendicular  to  the  surface,  it  continues  on 
in  a  straight  line  without  altering  its  course. 

2d.  When  light  passes  in  an  oblique  direction,  from  a 
7-arer  to  a  denser  medium,  it  will  be  turned  from  its 
course,  and  proceed  through  the  denser  medium  less 
obhquely,  and  in  a  line  nearer  to  a  perpendicular  to  its 
surface. 

3d.  When  light  passes  from  a  denser  to  a  rarer  medium, 
it  passes  through  the  rarer  medium  in  a  more  oblique 
direction,  and  in  a  line  further  from  a  perpendicular  to 
the  surface  of  the  denser  medium. 

1.  In  Fig.  110,  the  line  AB  repre- 
sents a  ray  of  light  passing  from  air  into 
water,  in  a  perpendicular  direction.  Ac- 
cording to  the  fiist  law,  stated  above,  it 
will  continue  on  in  the  same  line  through 
the  denser  medium  to  E.  If  the  ray 
were  to  pass  upward  through  the  denser 
medium,  the  water,  in  the  same  perpen- 
dicular direction  to  the  air,  by  the  same 
law  it  would  also  continue  on  in  the  same  straight  line  to  A. 

2.  But  if  the  ray  proceed  from  a  rarer  to  a  denser  medium, 
in  an  oblique  direction,  as  from  C  to  B,  when  it  enters  the 
denser  medium  it  will  not  continue  on  in  the  same  straight  line 
to  D,  but,  by  the  second  law,  stated  above,  it  will  be  refracted 
or  bent  out  of  its  course,  and  proceed  in  a  less  oblique  direction 
to  F,  which  is  nearer  the  perpendicular  ABE  than  D  is. 

3.  Again,  if  the  ray  proceed  from  the  denser  medium,  the 

have  a  greater  refracting  power,  and  are,  therefore,  called  denser  media 
than  water.  In  the  following  list,  the  various  substances  are  enumerated 
in  the  order  of  their  refractive  power,  or,  in  other  words,  in  the  order  of 
their  density,  as  media  ;  the  last-mentioned  being  the  densest,  and  the  first 
the  rarest,  namely:  air,  ether,  ice,  water,  alcohol,  alum,  olive  oil,  oil  of 
turpentine,  amber,  quartz,  glass,  melted  sulphur,  diamond. 


224.  What  are  the  three  fundamental  laws  of  Dioptrics?  Illustrate  the 
first  law  by  the  line  A  B,  in  Fig.  110.  Illustrate  the  second  law  by  the 
line  C  B.    Illustrate  the  third  law  by  the  Ime  FB. 


Fig.  110. 

G 


E  F 


OPTICS. 


100 


water,  to  the  rarer  medium,  the  air,  namely,  from  F  to  B, — 
instead  of  pursuing  its  straight  course  to  G,  it  will  be  refracted 
according  to  the  third  law  above  stated,  and  proceed  in  a  more 
oblique  direction  to  C,  which  is  further  fi'om  the  pei'pondiculcir 
E  B  A  than  G  is. 

4.  The  refraction  is  more  or  less  in  all  cases  in  proportion 
IS  the  rays  fall  more  or  less  obliquely  on  the  refracting  sur- 
I'ace. 

5.  From  what  has  now  been  stated,  with  regard  to  refrac- 
tion, it  will  be  seen  that  many  interesting  facts  may  be  explain- 
ed. Thus,  an  oar  or  a  stick,  when  partly  immersed  in  water, 
appears  bent,  because  we  see  one  part  in 'one  medium,  and  the 
other  in  another  medium  :  the  part  which  is  in  the  water  appears 
higher  than  it  really  is,  on  account  of  the  refraction  of  the 
denser  medium. 

6.  For  the  same  reason,  when  we  look  obliquely  upon  a  body 
Df  water  it  appears  more  shallow  than  it  really  is.  But  when 
we  look  imyendkularly  downwards,  we  are  liable  to  no  such 
deception,  because  there  will  be  no  refraction. 

7.  Let  a  piece  of  money  be  put  into  a  cup  or  a  bowl,  and 
^he  cup  and  the  eye  be  placed  in  such  a  position  that  the  side 
of  the  cup  will  just  hide  the  money  from  the  sight  ;  then  keep- 
ing the  eye  directed  to  the  same  spot,  let  the  cup  be  filled  with 
water, — the  money  will  become  distinctly  visible. 

225.  The  refraction  of  light  prevents  our  seeing  the 
heavenly  bodies  in  their  real  situation.* 

*  There  is  another  reason,  also,  why  we  do  not  see  the  heavenly  bodies 
in  their  true  situation.  Lis^ht,  thou^rh  it  move  with  great  velocity,  is  about 
eight  and  a  half  minutes  in  its  passage  from  the  sun  to  the  earth,  so  that 
when  the  rays  reach  us,  the  sun  has  quitted  the  spot  he  occupied  on  their 
departure  ;  yet  we  see  him  in  the  direction  of  those  rays,  and,  consequent- 
ly, in  a  situatioii  which  he  abandoned  eight  minutes  and  a  half  before.  The 
refraction  of  light  does  not  affect  the  appearance  of  the  heavenly  bodies 
when  they  are  vertical,  that  is,  directly  over  our  heads,  because  the  rays 
then  pass  vertically,  a  direction  incompatible  with  refraction. 

In  what  proportion  does  the  refraction  increase  or  diminish  ?  Why  does 
an  oar  or  a  stick,  when  partly  immersed  in  water,  appear  bent  ?  Why 
does  the  part  which  is  in  the  water  appear  higher  than  it  really  is?  Why 
does  a  body  of  water,  when  viewed  obliquely,  appear  more  shallow  than  it 
really  is?  In  what  direction  can  we  look  so  as  to  cause  no  refraction? 
What  experiment  is  here  related  ? 

225.  Why  do  we  not  see  the  heavenly  bodies  in  their  real  situation? 
8 


170 


NATURAL  PHILOSOPHY. 


The  light  which  they  send  to  us  is  refracted  in  passing 
through  the  atmosphere,  and  we  see  the  sun,  the  stars,  &c.,  in 
the  dii  ection  of  the  refracted  ray.  In  consequence  of  this  at- 
mospheric refraction  the  sun  sheds  his  hght  upon  us  earher  in 
the  morning  and  later  in  the  evening,  than  we  should  otherwise 
perceive  it.  And  when  the  sun  is  actually  below  the  horizon, 
those  rays  which  would  otherwise  be  dissipated  through  space, 
are  refracted  by  the  atmosphere  towards  the  surface  of  the 
earth,  causing  twihght.  The  greater  the  density  of  the  air  the 
higher  is  its  refractive  power,  and,  consequently,  the  longer  the 
duration  of  twilight. 

226.  When  a  ray  of  light  passes  from  one  medium  to 
another,  and  through  that  into  the  first  again,  if  the  two 
refractions  be  equal,  and  in  opposite  directions,  no  sen- 
sible effect  will  be  produced. 

This  explains  the  reason  why  the  refractive  power  of  flat 
window-glass  produces  no  effect  on  objects  seen  through  it. 
The  rays  sufl'er  two  refractions,  which,  being  in  contrary  di- 
rections, produce  the  same  effect  as  if  no  refraction  had  taken 
place. 

227.  A  lens  is  a  glass,  which,  owing  to  its  peculiar 
form,  causes  the  rays  of  light  to  converge  to  a  focus,  or 
disperses  them  according  to  the  laws  of  refraction. 

It  may  here  also  be  remarked,  that  it  is  entirely  owing  to  the  reflection  • 
of  the  atmosphere  that  the  heavens  appear  bright  in  the  daytime.  If  the 
atmosphere  had  no  reflective  power,  only  that  part  would  be  luminous  in 
which  the  sun  is  placed ;  and  on  turning  our  back  to  the  sun,  the  whole 
heavens  would  appear  as  dark  as  in  the  night ;  we  should  have  no  twi- 
light, but  a  sudden  transition  from  the  brightest  sunshine  to  darkness,  im- 
mediately upon  the  setting  of  the  sun. 


In  what  direction  do  we  see  them  ?  What  causes  twilight  ?  Upon 
what  does  the  duration  of  twilight  depend  ?  What  other  reason  is  given, 
in  the  note,  why  we  do  not  see  the  heavenly  bodies  in  their  true  situation? 
When  does  the  refraction  of  light  not  affect  the  appearance  of  the  heaven- 
ly bodies?    Why  do  the  heavens  appear  bright  in  the  daytime? 

226.  What  effect  is  produced  when  a  ray  of  light  passes  from  one  me- 
dium to  another,  and  through  that  into  the  first  again?  Why  does  the 
refractive  power  of  flat  window-glass  produce  no  effect  on  objects  seen 
through  it? 

227.  What  IS  a  lens? 


OPTICS. 


J71 


There  are  various  kinds  of  lenses,  named  according 
to  their  focus ;  but  they  are  all  to  be  considered  as  por- 
tions of  the  internal  or  external  surface  of  a  sphere. 

1.  A  single  con- 
vex lens  has  one  side 
flat  and  the  other 
convex ;  as  A  in  Fig. 
111. 

2.  A  single  con- 
cave lens  is  flat  on 
one  side  and  concave 
on  the  other,  as  B  in 
Fig.  111.  ' 

3.  A  double  convex  lens  is  convex  on  both  sides,  as  C, 
Fig.  111. 

4.  A  double  concave  lens  is  concave  on  both  sides,  as  D, 
Fig.  111. 

5.  A  meniscus^  is  convex  on  one  side  and  concave  on  the 
other,  as  E,  Fig.  111. 

6.  The  axis  of  a  lens  is  a  line  passing  through  the  centre ; 
thus,  F  G,  Fig.  Ill,  is  the  axis  of  all  the  five  lenses. 

228.  The  peculiar  form  of  the  various  kinds  of  lenses 
causes  the  light  which  passes  through  them  to  be  re- 
fracted from  its  course,  according  to  the  laws  of  Di- 
optrics. 

*  The  word  meniscus  is  derived  from  the  Greek  language,  and  means 
literally  a  little  moon.  This  term  is  applied  to  a  concavo-convex  lens, 
from  its  similarity  to  a  moon  in  its  early  appearance.  To  this  kind  of  lens 
the  term  periscopic  has  recently  been  applied,  from  the  Greek  language, 
meaning  literally  viewing  on  all  sides.  When  the  concave  and  convex 
sides  of  periscopic  glasses  are  even  or  parallel,  they  act  as  plane  glasses  ; 
but  when  the  sides  are  unequal,  or  not  parallel,  they  will  act  as  concave 
or  convex  lenses,  according  as  the  concavity  or  the  convexity  is  the  greater. 

How  are  all  lenses  to  be  considered?  What  is  a  single  convex  lens? 
What  part  of  Fig.  Ill  represents  a  single  convex  lens?  What  is  a  single 
concave  lens?  What  part  of  Fig.  Ill  represents  a  single  concave  lens? 
What  is  a  double  convex  lens?  What  part  of  Fig.  Ill  represents  a  double 
convex  lens?  What  is  a  double  concave  lens?  What  part  of  Fig.  Ill 
represents  a  double  concave  lens?  What  is  a  meniscus?  What  part  of 
Fig.  Ill  represents  a  meniscus?  What  is  the  axis  of  a  lens?  What  line, 
in  Fig.  Ill,  represents  the  axis  of  all  the  five  lenses? 

228.  What  is  stated  with  egard  to  the  form  of  the  lenses? 


172 


NATURAL  THILOSOPHY. 


It  will  be  remembered  that,  according  to  these  laws,  light, 
in  passing  from  a  rarer  to  a  denser  medium  is  refracted  to- 
wards the  perpendicular ;  and,  on  the  contrary,  that  in  pass- 
ing from  a  denser  to  a  rarer  medium,  it  is  refracted  further 
from  the  perpendicular.  In  order  to  estimate  the  effect  of  a 
lens,  we  must  consider  the  situation  of  the  perpendicular,  with 
respect  to  the  surface  of  the  lens.  Now,  a  perpendicular,  to 
any  convex  or  concave  surface,  must  always,  when  prolonged, 
pass  through  the  centre  of  sphericity ;  that  is,  in  a  lens,  the 
centre  of  the  sphere  of  which  the  lens  is  a  portion.  By  an  at- 
tentive observation,  therefore,  of  the  laws  above  stated,  and  of 
the  situation  of  the  perpendicular  on  each  side  of  the  lens,  it 
will  be  found  in  general, — 

1.  That  convex  lenses  collect  the  rays  into  a  focus,  and  mag- 
nify objects  at  a.  certain  distance. 

2.  That  concave  lenses  disjyerse  the  rays,  and  diminish  objects 
seen  through  them. 

229.  The  focal  distance  of  a  lens  is  the  distance  from 
the  middle  of  the  glass  to  the  focus.  This,  in  a  single 
convex  lens,  is  equal  to  the  diameter  of  the  sphere  of 
which  the  lens  is  a  portion  ;  and  in  a  double  convex 
lens  is  equal  to  the  radius  of  a  sphere  of  which  the  lens 
is  a  portion. 

230.  When  parallel  rays*  fall  on  a  convex  lens,  those 
only  which  fall  in  the  direction  of  the  axis  of  the  lens  are 
perpendicular  to  its  surface,  and  those  only  will  continue 
on  in  a  straight  line  through  the  lens.  The  other  rays, 
falling  obliquely,  are  refracted  towards  the  axis  and  will 
meet  in  a  focus. 

*  The  rays  of  the  sun  are  considered  parallel  at  the  surface  of  the  earth. 


How  is  light  refracted  in  passing  from  a  rarer  to  a  denser  medium  ? 
How,  in  passing  from  a  denser  to  a  rarer?  What  must  be  considered  in 
estimating  the  effect  of  lenses?  Through  what  must  a  perpendicular,  to 
any  convex  or  concave  surface,  always,  when  prolonged,  pass  ?  What  is 
stated  with  regard  to  convex  lenses?  What,  with  regard  to  concave 
lenses  ? 

229.  What  is  the  focal  distance  of  a  lens?  To  what  is  this  equal  in  a 
single  convex  lens  ?    To  what  is  it  equal  in  a  double  convex  lens  ? 

230.  When  parallel  rays  fall  on  a  convex  lens,  which  one  is  perpen- 
dicular to  its  surface  ?  How  are  the  other  rays,  falling  obliquely,  refracted  ? 


OPTICS. 


173 


It  is  tins  property  of  a  convex  lens  which  gives  it  its  power 
as  a  buniiiig-o-lass.  All  tlie  parallel  rays  of  the  sun  which  pass 
through  tlie  glass,  are  collected  together  in  the  focus;  and, 
consequently,  tJie  heat  at  the  focus  is  to  the  comm,on  heat  of  the 
SU71,  as  the  area  of  the  glass  is  to  the  area  of  the  focus.  Thus,  if 
a  lens,  four  inches  in  diameter,  collect  the  sun's  rays  into  a 
focus,  at  the  distance  of  twelve  inches,  the  image  will  not  be 
more  than  one-tenth  of  an  inch  in  diameter ;  the  surface  of  this 
little  circle  is  1600  times  less  than  the  surface  of  the  lens,  and, 
consequently,  the  heat  will  be  1600  times  greater  at  the  focus 
than  at  the  lens.^ 

231.  The  following  effects  result  from  the  laws  of  re 
fraction. 

FACTS  WITH  REGARD  TO  CONVEX  SURFACES. 

1.  Parallel  rays  passing  out  of  a  rarer  into  a  denser  medium, 
through  a  convex  surface,  will  become  converging. 

2.  Diveiging  rays  will  be  made  to  diverge  less,  to  become 
parallel,  or  to  converge,  according  to  the  degree  of  divergenc}?- 
before  refraction,  or  the  convexity  of  the  surface. 

3.  Converging  rays,  towards  the  centre  of  convexity,  will 
suffer  no  refraction. 

4.  Rays  converging  to  a  point  beyond  the  centre  of  con- 
vexity, will  be  made  more  converging. 

*  The  following  effects  were  produced  by  a  large  lens,  or  burning-glass, 
two  feet  in  diameter,  made  at  Leipsic  in  1691.  Pieces  of  lead  and  tin 
were  instantly  melted  ;  a  plate  of  iron  was  soon  rendered  red-hot,  and 
afterwards  fused,  or  melted  ;  and  a  burnt  brick  was  converted  into  yellow 
glass.  A  double  convex  lens,  three  feet  in  diameter,  and  weighing  212 
pounds,  made  by  Mr.  Parker,  in  England,  melted  the  most  refractory  sub- 
stances. Cornelian  was  fused  in  75  seconds,  a  crystal  pebble  in  6  seconds, 
and  a  piece  of  white  agate  in  30  seconds.  This  lens  was  presented  by  the 
king  of  England  to  the  emperor  of  China. 

What  property  of  a  convex  lens  gives  it  its  power  as  a  burning-glass  ? 
Where  are  all  the  parallel  rays  of  the  sun,  which  pass  through  the  glass, 
collected?  How  does  the  heat  at  the  focus  compare  with  the  common 
heat  of  the  sun  ?  What  is  related  in  the  note  with  regard  to  the  effects  of 
lenses  produced  by  burning-glasses? 

231.  What  is  the  first  effect  related  as  resulting  from  the  laws  of  re- 
fraction with  regard  to  convex  surfaces?  What  is  said  of  diverging  rays? 
What  is  said  of  rays  converging  towards  the  centre  of  convexity  ?  What 
of  rays  converging  to  a  point  beyond  the  centre  of  convexity? 


174  NATURAL  PHILOSOPHY. 

5  Convemno-  rays  towards  a  point  nearer  the  surface  than 
the  centre  of  convexity,  will  be  made  less  converging  by  refrac- 

*'^[When  the  rays  proceed  out  of  a  denser  into  a  ra^'er  medium, 
the  reverse  occurs  in  each  case.] 

FACTS  WITH  REGARD  TO  CONCAVE  SURFACES. 

1.  Parallel  rays,  proceeding  out  of  a  rarer  into  a  denser 
medium,  through  a  concave  surface,  are  made  to  diverge. 

2  Diverging  rays  are  made  to  diverge  more,— to  suher  no 
refraction,— or  to  diverge  less,  according  as  they  proceed  from 
a  point  beyond  the  centre,  from  the  centre,  or  between  the 
centre  and  the  surface.  . 

3  Converging  rays  are  made  less  convergmg,  parallel,  or 
diverging,  according  to  their  degree  of  convergency  before  re- 
fraction.^ .  ,. 

[When  the  rays  proceed  out  of  a  denser  mto  a  rarer  medmm, 
the  reverse  takes  place  in  each  case.] 

232.  Double  convex,  and  doable  concave  glasses,  or 
lenses,  are  used  in  spectacles,  to  remedy  the  defects  ol 
the  eve  ;  the  former,  when  by  age  it  becomes  too  iiat,  or 
loses'  a  portion  of  its  roundness  ;  the  latter,  when  by 

*  The  above  eight  principles  are  all  the  necessary  consequence  of  the 
operation  of  the  three  laws  mentioned  as  the  fundamental  laws  of  Dioptrics.. 
The  reason  that  so  many  different  principles  are  produced  by  the  operation 
of  those  laws,  is,  that  the  perpendiculars  to  a  convex  or  concave  surface 
are  constantly  varying,  so  that  no  two  are  parallel.  But  m  flat  surfaces 
the  perpendiculars  are  parallel;  and  one  invariable  result  is  produced  by 
the  rays  when  passing  from  a  rarer  to  a  denser,  or  from  a  denser  to  a 
rarer  medium,  havuig  a  flat  surface. 

What  of  ravs  converging  to  a  point  nearer  the  surface  than  the  centre 
of  convexity?  '  When  the  rays  proceed  out  of  a  denser  into  a  rarer  me- 
dium, what  occurs?  . 

What  is  stated  of  parallel  rays,  proceeding  from  a  rarer  mto  a  denser 
medium,  through  a  concave  surface?  What  is  said  of  diverging  rays? 
What  is  said  of  converging  rays?  Of  what  are  the  above  eight  prmci- 
ples  the  necessary  consequence  ?  What  is  the  reason  that  so  -many  dif- 
ferent principles  are  produced  by  the  operation  of  these  laws  ?  ^ 

232.  For  what  are  double  convex  and  concave  glasses,  or  lenses,  used  in 
spectacles  ? 


OPTICS. 


175 


any  other  cause  it  assumes  too  round  a  form,  as  in  the 
case  of  short-sighted  (or,  as  they  are  sometimes  called, 
near-sighted)  persons.  Convex  ghvsses  are  used  when 
the  eye  is  too  flat,  and  concave  glasses  w^hen  it  is  too 
round.* 


Fig.  112. 


THE  EYE. 

233.  The  eye  is  composed  of  a  number  of  coats,  or 
coverings,  w^ithin  which  are  enclosed  a  lens,  and  certain 
humors,  in  the  shape,  and  performing  the  office  of  con 
vex  lenses. 

1.  The  different  parts  of  the  eye,  are: 

1.  The  Cornea.  6.  The  Vitreous  Humor. 

2.  The  Iris.  7.  The  Retina. 

3.  The  Pupil.  8.  The  Choroid. 

4.  The  Aqueous  Humor.      9.  The  Sclerotica. 

5.  The  Crystalline  Lens.     10.  The  Optic  Nerve. 

2.  Fig.  112  represents  a  front 
view  of  the  eye,  in  which  a  a  rep- 
resents the  Go  nea,  or,  as  it  is 
commonly  called,  the  white  of  the 
eye  ;  eei^  the  Iris,t  having  a  cir- 
cular opening  in  the  centre,  called 
the  pupil,  2h  which  contracts  in  a 
strong  light,  and  expands  in  a  faint 
light,  and  thus  regulates  the  quan- 
tity which  is  admitted  to  the  ten- 
der parts  in  the  interior  of  the  eye. 

^  These  lenses  or  glasses  are  generally  numbered,  by  opticians,  accord- 
ing to  their  degree  of  convexity  or  concavity  ;  so  that  by  knowing  the 
number  that  fits  the  eye,  the  purchaser  can  generally  be  accommodated, 
without  the  trouble  of  trying  many  glasses. 

t  It  is  the  iris  which  gives  the  peculiar  color  to  the  eye. 


What  glasses  are  used  when  the  eye  is  too  flat  ?  What  are  used  when 
the  eye  is  too  round? 

233.  Of  what  is  the  eye  composed?  What  are  the  different  parts  of 
the  eye?  First?  Second?  Third?  Fourth?  Fifth?  Sixth?  Seventh? 
Eighth?  Ninth?  Tenth?  What  does  Fig.  1 12  represent ?  Explain  tho 
figure. 


176 


NATURAL  PHILOSOPHY. 


3.  Fig.  113  represents  a  side 
view  of  the  eye,  laid  open,  in 
which  h  b  represents  the  cor- 
nea, e  e  the  his,  d  d  the  pupil, 
//  the  aqueous  humor,  gg  the 
crystalline  lens,  hh  the  vitreous 
liumor,  i  i  i  i  i  the  retina,  c  c 
the  choroid,  aaaaa  the  scle- 
] -otic a,  and  n  the  optic  nerve. 

4.  The  cornea  forms  the 
anteiior  portion  of  the  eye. 
It  is  set  in  the  sclerotica  in  the 

same  manner  as  the  crystal  of  a  watch  is  set  in  the  case.  Its 
degree  of  convexity  varies  in  different  individuals  and  in  differ- 
ent periods  of  life.  As  it  covers  the  pupil  and  the  iiis,  it  pro- 
tects them  from  injury.  Its  principal  office  is  to  cause  the 
light  which  reaches  the  eye  to  converge  to  the  axis.  Part  of 
the  light,  however,  is  reflected  by  its  finely  pohshed  surface, 
and  causes  the  brilliancy  of  the  eye. 

5.  The  iris  is  so  named  from  its  being  of  different  colors.  It 
is  a  kind  of  circular  curtain,  placed  in  the  front  of  the  eye  to 
regulate  the  quantity  of  light  passing  to  the  back  part  of  the 
eye.  It  has  a  circular  opening  in  the  centre,  which  it  involun- 
tarily enlarges  or  diminishes. 

6. "  The  pupil  is  merely  the  opening  in  the  iiis,  through  which 
the  light  passes  to  the  lens  behind.  It  is  always  circular  in 
the  human  eve,  but  in  quadrupeds  it  is  of  different  shape. 
When  the  pupil  is  expanded  to  its  utmost  extent,  it  is  capable  " 
of  admitting  ten  times  the  quantity  of  light  that  it  does  when 
most  contracted.^    In  cats  and  other  animals,  which  are  said 

*  When  we  come  from  a  dark  place  into  a  strong  our  eyes 

snfier  pain,  because  the  pupil  being  expanded,  admits  a  larger  quantity  of 
light  to  rush  in,  before  it  has  bad  time  to  contract.  And  when  we  ofo 
from  a  strong  hght  into  a  faint  one,  we  at  first  imaofine  ourselves  in  dark- 
ness, because  the  piipil  is  then  contracted,  and  does  not  inxtaniJy  expand. 

What  does  Fig.  113  represent?  Explain  the  figure.  \Vnat  part  of  the 
eye  does  the  cornea  form  ?  Is  its  degree  of  convexity  the  same  m  ail  per 
sons  and  all  periods  of  life  ?  What  is  its  principal  office  ]  From  what 
does  the  iris  take  its  name  ?  What  is  the  use  of  the  iris  ?  What  is  the 
pupil  ?  W^hat  is  its  form  in  the  human  eye  ?  How  much  more  light  is 
the  pupil  capable  of  admitting,  when  expanded  to  its  utmost  extent,  thai 
when  most  contracted  ? 


OPTICS. 


177 


to  see  in  the  dark,  the  power  of  dikitation  and  contraction  is 
much  i^: eater;  it  is  computed,  tliat  their  pupils  may  receive 
one  bundled  times  more  light  at  one  time  than  at  another. 
Tbat  bo  bt  only,  which  passes  the  pupil,  can  be  of  use  in  vision  ; 
that  which  falls  on  the  iris  being  reflected,  returns  through 
the  cornea,  and  exhibits  the  color  of  the  iris. 

7.  The  aqueous  humor  is  a  fluid,  as  clear  as  the  purest 
water.  In  shape  it  resembles  a  meniscus,  and,  being  situated 
between  the  cornea  and  the  crystalline  lens,  it  assists  in  collect- 
ing and  transmitting  the  rays  of  light  from  external  objects  to 
that  lens. 

8.  The  crystalline  lens  is  a  transparent  body,  in  the  form  of 
a  double  convex  lens,  placed  between  the  aqueous  and  vitreous 
humors.  Its  office  is  not  only  to  collect  the  rays  to  a  focus, 
on  the  retina,  but  also  to  increase  the  intensity  of  the  light 
which  is  directed  to  the  back  part  of  the  eye. 

9.  The  vitreous  humor  (so  called  from  its  resemblance  to 
melted  glass)  is  a  perfectly  transparent  mass,  occupying  the 
globe  of  the  eye.  Its  shape  is  like  a  meniscus,  the  convexity 
of  which  greatly  exceeds  the  concavity. 

10.  In  Fig.  114  the  shape  of  the  Fig.  114. 
aqueous  and  vitreous  humors  and  the 
crystalline  lens  is  presented,  a  is  the 
aqueous  humor,  which  is  a  meniscus,  h 
the  crystalhne  lens,  which  is  a  double  " 
convex  lens,  and  c  the  vitreous  humor, 
which  is,  also,  a  meniscus,  whose  con- 
cavity has  a  smaller  radius  than  its 
convexity. 

11.  The  retina  is  the  seat  of  vision.  The  rays  of  light  being 
refracted  in  their  passage  by  the  other  parts  of  the  eye,  are 
brought  to  a  focus  in  the  retina,  where  an  inverted  image  of  the 
object  is  represented. 

12.  The  choroid  is  the  inner  coat  or  covering  of  the  eye.  Its 
outer  and  inner  surface  is  covered  with  a  substance  called  the 


What  is  said  of  those  animals  which  are  said  to  see  in  the  dark?  What 
light,  only,  is  of  use  in  vision?  What  becomes  of  the  light  which  falls  on 
the  iris?  What  is  the  aqueous  humor?  What  is  its  form?  Of  what  use 
is  it?  What  is  the  crystalhne  lens?  What  is  its  office?  What  is  the 
vitreous  humor  ?  Why  do  persons  sometimes  experience  pain  when  pass- 
ing from  a  dark  place  into  strong  light  ?  What  is  the  shape  of  the  vitreous 
humor?  Explain  Fig  114.  What  is  the  retina?  What  is  the  choroid? 
8* 


178 


NATURAL  PHILOSOPHY. 


mqmentum  nigrum,  (or  black  paint.)  Its  office  is,  apparently, 
to  absorb  the  rays  of  light  immediately  after  they  have  fal  en 
on  the  retina.  It  is  the  opinion  of  some  philosophers,  that  it 
is  the  choroid  and  not  the  retina,  which  conveys  the  sensation 
produced  by  rays  of  light  to  the  brain.  _ 

13  The  sclerotica  is  the  outer  coat  of  the  eye.  It  derives 
its  name  from  its  hardness.  Its  office  is  to  preserve  the 
lobular  figure  of  the  eye,  and  defend  its  more  delicate  internal 
structure  To  the  sclerotica  are  attached  the  muscles  which 
move  the  eye.  It  receives  the  cornea,  which  is  inserted  m  it 
somewhat  like  a  watch-glass  in  its  case.  It  is  pierced  by  the 
optic  nerve,  which,  passing  through  it,  expands  over  the  inner 
surface  of  the  choroid,  and  thus  forms  the  retina. 

14  The  optic  nerve  is  the  organ  which  carries  the  impres- 
sions made  by  the  rays  of  light,  (whether  by  the  medium  of  the 
retina,  or  the  choroid,)  to  the  brain,  and  thus  produces  the 
sensation  of  sight.^ 

234.  The  eye  is  a  natural  camera  ohscura,\  and  the 
images  of  all  objects  seen  bv  the  eye  are  represented  on 
the  retina,  in  the  same  manner  as  the  forms  of  external 
objects  are  delineated  in  that  instrument. 

1  Fio-  115  represents  only  those  parts  of  the  eye  which 
are  mos't  essential  for  the  explanation  of  the  phenomena  of 

Fig.  115. 


*  For  the  above  description  of  the  eye  and  its  parts,  the  author  is  main- 
ly indebted  to  Paxton's  Introduction  to  the  Study  of  Anatomy,  edited  by 
Dr.  Lewis  of  this  city. 

t  The  camera  obscura  is  explained  in  a  note  on  page  157. 

Bv  what  is  its  outer  and  inner  surface  covered?  What  is  its  office? 
What  is  the  opinion  of  some  philosophers  with  regard  to  the  cnoroid? 
What  is  the  sclerotica  ?  From  what  does  it  derive  its  name  ?  What  is  its 
office  ?    What  are  attached  to  the  sclerotica  ?    What  is  the  optic  nerve  ? 


OPTICS. 


179 


vision.  The  image  is  formed  thus.  The  rays  from  the  object 
c  d,  diverging  towards  the  eye,  enter  the  cornea  c,  and  cross  one 
another  in  their  passage  through  the  crystaUine  lens  d,  by 
which  they  are  made  to  converge  on  the  retina,  where  they 
form  the  inverted*  image,  fe. 

2.  The  convexity  of  the  crystalhne  humor  is  increased  or 

*  Although  the  image  is  inverted  on  the  retina,  we  see  objects  erect, 
because  all  the  images  formed  on  the  retina  have  the  same  relative  position 
which  the  objects  themselves  have ;  and  as  the  rays  all  cross  each  other, 
the  eye  is  directed  upwards,  to  receive  the  rays  w^hich  proceed  from  the 
upper  part  of  an  object,  and  downwards,  to  receive  those  which  proceed 
from  the  lower  part. 

A  distinct  image  is  also  formed  on  the  retina  of  each  eye  ;  but  as  the 
optic  nerves  of  the  two  eyes  unite,  or  cross  each  other  before  they  reach 
the  brain,  the  impressions  received  by  the  two  nerves  are  united,  so  that 
only  one  idea  is  excited,  and  objects  are  seen  single.  Although  an  object 
may  be  distinctly  seen  with  only  one  eye,  it  has  been  calculated  that  the 
use  of  both  eyes  makes  a  difference  of  about  one-twelfth.  From  the  de- 
scription now  given  of  the  eye,  it  may  be  seen  what  are  the  defects  which 
are  remedied  by  the  use  of  concave  and  convex  lenses,  and  how  the  use  of 
these  lenses  remedies  them.  When  the  crystalline  humor  of  the  eye  is 
too  round,  the  rays  of  light  which  enter  the  eye  converge  to  a  focus 
before  they  reach  the  retina,  and,  therefore,  the  image  will  not  be  distinct ; 
and  when  the  crystalline  humor  is  too  flat,  (as  is  often  the  case  with  old 
persons,)  the  rays  will  not  converge  on  the  retina,  but  tend  to  a  point 
beyond  it.  A  convex  glass,  by  assisting  the  convergency  of  the  crystal- 
line lens,  brings  the  rays  to  a  focus  on  the  retina,  and  produces  distinct 
vision. 

The  eye  is  also  subject  to  imperfection  by  reason  of  the  humors  losing 
their  transparency,  either  by  age  or  disease.  For  these  imperfections  no 
glasses  offer  a  remedy  without  the  aid  of  surgical  skill.  The  operation  of 
couching  and  removing  cataracts  from  the  eye,  consists  in  making  a  punc- 
ture or  incision  through  which  the  diseased  part  may  escape.  Its  office  is 
then  supplied  by  a  lens.  If,  however,  the  operator,  by  accident  or  want 
of  skill,  permit  the  vitreous  humor  to  escape,  the  globe  of  the  eye  imme- 
diately diminishes  in  size,  and  total  blindness  is  the  inevitable  result. 


234.' What  philosophical  instrument  does  the  eye  resemble  in  its  con- 
struction? Explain  Fig.  115.  Note.  Why  do  the  objects  appear  erect 
when  the  images  are  inverted  ?  Why  do  we  see  only  one  image  when 
an  image  is  formed  on  both  eyes?  What  are  the  defects  which  are 
remedied  by  the  use  of  concave  and  convex  lenses  ?  In  what  other  way 
is  the  eye  subject  to  imperfection  ?    Is  there  any  remedy  for  this  ? 


ISO 


NATURAL  PHILOSOPHY. 


diminished  bv  means  of  two  muscles,  to  yvhich.  it  is  attacbed. 
Bv  this  means  the  focus  of  the  rays  which  pass  through  it, 
constantlv  falls  on  the  retina ;  and  an  equally  disimct  image  is 
formed,  both  of  distant  objects  and  those  which  are  near. 

235.  A  single  microscope  consists  simply  of  a  convex 
lenN  commonly  called  a  magnifying-glass ;  in  the  locus 
of  which  the  object  is  placed,  and  through  which  it  is 
viewed. 

1.  By  means  of  a  microscope  the  rays  of  hght  from  an  ob- 
ject are  caused  to  divero-e  less ;  so  that  when  they  enter  the 
pupil  of  the  eye.  they  Ml  parallel  on  the  crystalhne  lens,  by 
which  thev  are 'refracted  to  a  focus  on  the  retina. 

2.  Fig. 'lie  represents  a  convex  lens,  or  single  microscope, 
C  P.  The  diverging 

rays  from  the  ob-  Fig.  lie. 

ject  AB  are  refract- 
ed in  their  passage 
throuo'h  the  lens 
C  P,  Imd  made  to 
fall  parallel  on  the 
crystalline  lens,  by 
which  they  are  re- 
fracted to  a  focus 
on  the  retina  R  R  ; 
and  the  image  is 

thus  magnified,  because  the  divergent  rays  are  collected  by  the 
lens  and  carried  to  the  retina. 

3.  Those  lenses  or  microscopes  which  have  the  shortest 
focus,  have  the  greatest  m:i unifying  power;  and  those  which 
are  the  most  bulging  or  convex,  have  the  shortest  focus. 
Lenses  are  made  small  because  a  reduction  in  size  is  necessary 
to  an  increase  of  curvature. 

•236.  A  double  microscope  consists  of  two  convex 
lenses,  bv  one  of  which  a  magnified  image  is  iormed, 


By  what  is  the  convexity  of  the  crystalhne  humor  increased  or  dimin- 
ished ?    ^^'hat  is  effected  by  this  means  1 

235.  What  is  a  single  microscope  '  What  is  the  use  of  this  microscope  ? 
What  fi2:ure  represents  a  microscope  ?  Explain  the  figure.  What  lenses 
have  the  greatest  magnifying  power'  What  lenses  have  the  shortest 
focus  ? 

Q36.  Of  v>'hat  does  a  double  microscope  consist  ? 


OPTICS. 


181 


and  by  the  other  this  image  is  carried  to  the  retina  of 
the  eye. 

Kig\  117  represents  the  effect  produced  by  the  lenses  of  a 
double  microscope.  The  rays  wliich  diverge  from  the  object 
A  13  are  collected  by  the  lens  L  M,  (called  the  object-glass,  be- 
cause it  is  nearest  to  the  object,)  and  form  an  inverted  magni- 

Fig.  117. 


fied  image  at  C  D.  The  rays  which  di veige  from  this  image 
are  collected  by  the  lens  N  0,  (called  the  eye-glass,  because 
it  is  nearest  to  the  eye,)  which  acts  on  the  principle  of  the 
single  microscope,  and  forms  a  still  more  magnified  image  on 
the  retina  R  R. 

237.  The  solar  microscope,  is  a  microscope  with  a 
mirror  attached  to  it,  upon  a  moveable  joint,  which  can 
be  so  adjusted  as  to  receive  the  sun's  rays  and  reflect 
them  upon  the  object.  Jt  consists  of  a  tube,  a  mirror  or 
looking-glass,  and  two  convex  lenses.  The  sun's  rays 
are  reflected  by  the  mirror  through  the  tube  upon  the 
object ;  the  image  of  which  is  thrown  upon  a  white 
screen,  placed  at  a  distance  to  receive  it. 

1.  The  microscope,  as  above  described,  is  used  for  viewing 
transparent  objects  only.  When  opaque  objects  are  to  be 
viewed,  a  mirror  is  used  to  reflect  the  light  on  the  side  of  the 


What  is  the  use  of  these  two  lenses?  What  does  Fig.  117  represent? 
Explain  the  figure. 

237.  What  is  the  solar  microscope?  Of  what  does  it  consist?  By 
what,  in  this  microscope,  are  the  sun's  rays  reflected,  and  upon  what? 
For  viewing  what  objects,  only,  is  the  microscope,  above  described,  used? 
How  do  those  microscopes,  used  for  viewing  opaque  objects,  differ  from 
these  ? 


182 


NATURAL  PHILOSOPHY. 


object ;  the  image  is  then  formed  by  light  reflected  from  the 
object,  instead  of  being  transmitted  through  it. 

2.  The  mao-nifying  power  of  a  single  microscope  is  ascertain- 
ed by  dividing  the  least  distance  at  which  an  object  can  be  dis- 
tinctly seen  by  the  naked  eye,  by  the  focal  distance  of  the  lens. 
This,  in  common  eyes,  is  about  7  inches.  Thus,  if  the  focal 
distance  of  a  lens  be  only  \  of  an  inch,  then  the  diameter 
of  an  object  will  be  magnified  28  times,  (because  7,  divided  by 
i,  is  the  same  as  multiplying  7  by  4,)  and  the  mrface  will  be 
magnified  784  times. 

3.  The  magnifying  power  of  the  compound  microscope 
found  in  a  similar  manner,  by  ascertaining  the  magnifying 
power,  first  of  one  lens,  and  then  of  the  other. 

4.  The  magnifying  power  of  the  solar  microscope  is  in  pro- 
portion as  the  distance  of  the  image,  from  the  object-glass,  is 
greater  than  that  of  the  object  itself  from  it.  Thus,  if  the  dis- 
tance of  the  object  from  the  object-glass  be  \  of  an  inch,  and 
the  distance  of  the  image>  or  picture,  on  the  screen,  be  ten 
feet,  or  120  inches,  the  object  will  be  magnified  in  length  480 
times,  or,  in  surface,  230,000  times.^ 

238.  The  magic  lantern  is  an  instrument  constructed 
on  the  principle  of  the  solar  microscope,  but  the  light  is 
supplied  by  a  lamp  instead  of  the  sun. 

1.  The  objects  to  be  viewed  by  the  magic  lantern  are 
generally  painted  with  transparent  colors,  on  glass  shdes,  which 
are  received  into  an  opening  in  the  front  of  the  lantern.  The 
light  from  the  lamp,  in  the  lantern,  passes  through  them,  and 
carries  the  pictures,  painted  on  the  shdes,  through  the  lenses, 
by  means  of  which  a  magnified  image  is  thrown  upon  the  wall, 
on  a  white  surface  prepared  to  receive  it. 

*  A  lens  may  be  caused  to.  magnify  or  to  diminish  an  object.  If  the 
object  be  placed  at  a  distance  from  the  focus  of  a  lens,  and  the  image  be 
formed  in  or  near  the  focus,  the  image  will  be  diminished  ;  but  if  the  ob- 
ject be  placed  near  the  focus,  the  imago  will  be  magnified. 


How  is  the  image  then  formed?  How  is  the  magnifying  power  of  a 
single  microscope  ascertained?  Illustrate  this.  How  is  the  magnifying 
of  the  compound  microscope  ascertained?  In  what  proportion  is  the  mag- 
nifying power  of  the  solar  microscope  ?  Illustrate  this.  iYo^e.  How  may 
a  lens  be  made  to  magnify  or  diminish  an  object  ? 

238.  What  is  the  magic  lantern  ?  How  are  objects,  viewed  by  the  magic 
lantern,  generally  represented  ? 


OPTICS. 


183 


2.  Fig.  118  represents  the  magic  limtern.  The  rays  of  hght 
from  the  lamp  are  received  upon  the  concave  mirror  e,  and  re- 


Fig.  118. 


fleeted  to  the  convex  lens  c,  which  is  called  the  condensing  lens, 
because  it  concentrates  a  large  quantity  of  light  upon  the  object 
painted  on  the  shde,  inserted  at  h.  The  rays  from  the  illu- 
minated object  at  h,  are  carried  divergent  through  the  lens  a, 
forming  an  image  on  the  screen  at  /.  The  image  will  increase 
or  diminish  in  size,  in  proportion  to  the  distance  of  the  screen 
from  the  lens  a. 

239.  A  telescope  is  an  instrument  for  viewing  distant 
objects. 

There  are  two  kinds  of  telescopes,  namely,  the  refract- 
ing telescope  and  the  reflecting  telescope. 

A  refracting  telescope  is  one  in  which  the  object  itself 
is  viewed,  through  the  medium  of  a  number  of  lenses. 

A  reflecting  telescope  is  one  in  which  the  image 
of  the  object  is  reflected  from  a  concave  mirror,  within 
the  tube  of  the  telescope,  and  viewed  through  a  number 
of  lenses.* 

*  The  image  of  the  object  seen  through  a  refracting  telescope  is  never 
so  clear  and  perfect  as  that  obtained  by  the  reflecting  telescope  ;  because 

What  figure  represents  a  magic  lantern?  Explain  the  figure.  In  what 
proportion  will  the  size  of  the  image  increase  or  diminish  ? 

239.  What  is  a  telescope?  How  many  kinds  of  telescopes  are  there? 
What  are  they  ?  What  is  a  refracting  telescope  ?  What  is  a  reflecting 
telescope?  Note.  Why  is  the  image  of  an  object,  seen  through  a  refract- 
ing telescope,  less  clear  and  perfect  than  when  seen  through  a  reflecting 
telescope  ? 


184 


NATURAL  PHILOSOPHY. 


1.  There  are  two  kinds  of  refracting  telescopes,  called  the 
astronomical  telescope,  or  night-glass,  and  the  terrestrial  tele- 
scope, or  day-glass.^  In  the  former,  or  night-glass,  there  are 
but  two  lenses  or  glasses,  but  the  object  is  viewed  in  an  invert- 
ed position.  As  the  glass  is  used  principally  for  viewing  the 
heavenly  bodies,  the  inversion  of  the  image  produces  no  incon- 
venience. In  the  latter,  or  day-glass,  two  additional  lenses  are 
introduced  to  give  the  image  its  natural  position. 

2.  Fig.  119  represents  a  night-glass,  or  astronomical  tele- 
scope. It  consists  of  a  tube,  A  B  C  D,  containing  two  glasses, 
or  lenses.  The  lens,  A  B,  having  a  longer  focus,  forms  the 
object-glass ;  the  other  lens,  D  C,  is  the  eye-glass.    The  rays 


Fig.  119. 

..___A   M 


from  a  very  distant  body,  as  a  star,  and  which  may  be  con- 
sidered parallel  to  each  other,  are  refracted  by  the  object-glass 
A  B  to  a  focus  at  K.  The  image  is  then  seen  through  the  eye- 
glass D  C,  magnified  as  many  times  as  the  focal  leng^.h  of  the 
eye-glass  is  contained  in  the  focal  length  of  the  object-glass. 
Thus,  if  the  focal  length  of  the  eye-glass  D  C,  be  contained 
100  times  in  that  of  the  object-glass  A  B,  the  star  will  be  seen 
magnified  100  times.  It  will  be  seen  by  the  figure,  that  the 
image  is  inverted ;  for  the  ray  M  A,  after  refraction,  will  be 
seen  in  the  direction  C  0,  and  the  ray  N  B,  in  the  direction  D  P. 

3.  Fig.  120  represents  a  day-glass  or  terrestrial  telescope, 
commonly  called  a  spy-glass.  This,  likewise,  consists  of  a 
tube,  A  B  H  G,  containing  four  lenses,  or  glasses,  namety, 

the  dispersion  of  colors  which  every  lens  produces,  in  a  greater  or  less  de- 
gree, renders  the  image  dull  and  indistinct,  in  proportion  to  the  number  of 
lenses  employed. 

*  Some  glasses  or  telescopes  are  marked  "  Night  and  Day."  These 
have  four  glasses,  two  of  which  maybe  removed  when  the  heavenly  bodies 
are  viewed. 


How  many  kinds  of  refracting  telescopes  are  there  ?  What  are  they  ? 
How  do  they  differ  the  one  from  the  other?  What  does  Fig.  119  repre- 
sent ?    Explain  the  figure. 


OPTICS. 


185 


A  1^  C  1),  K  \'\  iun\  Ct  If.  The  Iciis  A  15  is  the,  ()l)ject-<rl;iss, 
jiiul    (J  11  ('V('-l;I;iss.      The   (wo   additional  (^ye-glass(^s, 

K  K  and  C  1),  arc  ol"  I  ho  sanu^  size  and  shape,  and  placed 

Fig.  120. 


at  e(|iial  distances  from  each  other,  in  such  a  manner  that  the 
focus  of  tlie  one  meets  that  of  the  next  lens.  These  two  eye- 
glasses, EF  and  CD,  are  introduced  for  the  purpose  of  col- 
lecting the  rays  proceeding  from  the  inverted  image  M  N,  into 
a  new  upright  image,  between  G  H  and  E  F,  and  the  image  is 
then  seen  tlirough  the  last  eye-glass  G  H,  under  the  angle  of 
vision,  P  0  Q. 

4.  Fig.  121  represents  a  reflecting  telescope.  This  consists 
also  of  a  large  tube,  containing  two  concave  metallic  mirrors, 
A  B  and  C,  with  two  plano-convex  eye-glasses.    The  mirrors 


Fig.  121. 


are  placed  at  a  little  more  than  the  sum  of  their  focal  distance 
from  each  other.  The  parallel  rays  r  r,  coming  from  a  distant 
object,  are  reflected  to  a  focus  g,  by  the  concave  mirror  A  B, 
and  thus  form  an  inverted  image  at  (/;  the  diverging  rays  pro- 
ceeding from  this  image  are  again  reflected  by  the  small  mirror 
C,  and  received  by  the  eye-glass  F,  through  an  aperture  in  the 
middle  of  the  mirror  A  B.  The  eye-glass  F  collects  these  re- 
flected rays  into  a  new  image  at  I,  and  this  image  is  seen 
magnified  through  the  second  eye-glass,  G. 


What  does  Fig.  120  represent  ?  Explain  the  figure.  What  does  Fig. 
121  represent?    Explain  the  figure. 


186 


NATURAL  PHILOSOPHY. 


5.  In  reflecting  telescopes,  mirrors  are  used  to  bring  the 
image  near  the  eye  ;  and  a  lens,  or  eye-glass,  is  employed  to 
magnify  the  image. 

6.  The  advantage  of  reflecting  telescopes  is,  that  they  pos- 
sess greater  magnifying  power,  and  do  not  decompose  the 
light.^ 

240.  That  part  of  the  science  of  Optics  which  relates 

to  colors  is  called  Chromatics. 

Colors  do  not  exist  in  the  bodies  themselves,  but  are  caused 
by  the  pecuhar  manner  in  which  the  light  is  reflected  from 
their  surfaces. 

241.  Light  is  composed  of  rays  of  different  colors, 
which  may  be  separated  by  a  prism. f 

242.  A  prism  is  a  solid  triangular,  or  three-sided 
piece  of  highly-poUshed  glass,  generally  six  or  eight 
inches  long.J 

243.  The  colors  which  enter  into  the  composition  of 
light  are  seven,  namely,  red,  orange,  yellow,  green, 

*  Common  telescopes  have  a  defect  arising  from  the  convexity  of  the 
object-glass,  which,  as  it  is  increased,  has  a  tendency  to  tinge  the  edges 
of  the  images.  To  remedy  this  defect,  achromatic  lenses  were  formed  by 
tiie  union  of  a  convex  lens  of  crown  glass  with  a  concave  lens  of  flint 
glass.  Owing  to  the  difference  of  the  refracting  power  of  these  two  kinds 
of  glass,  the  images  become /ree  from  color  and  more  distinct,  and  hence 
the  glasses  which  produce  them  were  called  achromatic,  that  is^  free  from 
color. 

t  This  discovery  was  made  by  Sir  Isaac  Newton. 

X  A  prism  may  be  made  of  three  pieces  of  plate  glass,  about  six  or  eight 
inches  long,  and  two  or  three  broad,  joined  together  at  their  edges,  and 
made  water-tight  by  putty.  The  ends  may  be  fitted  to  a  triangular  piece 
of  wood,  in  one  of  which  an  aperture  is  made  by  which  to  fill  it  with  water, 
and  thus  to  give  it  the  appearance  and  the  refractive  power  of  a  solid 
prism. 

Why  are  mirrors  used  in  reflecting  telescopes?  What  is  the  use  of  the 
lens?    W^hat  is  the  advantage  of  the  reflecting  telescope  ? 

240.  What  is  Chromatics?    What  causes  color? 

241.  Of  what  is  light  composed?  How  can  these  rays  be  separated? 
Note.  By  whom  was  this  discovery  made? 

242.  What  is  a  prism  ?    Note.  How  may  a  prism  be  made? 

243.  How  many  colors  enter  into  the  composition  of  light?  What  are 
they  ? 


OPTICS. 


187 


blue,  indigo,  and  violet.  Each  of  these  has  a  different 
degree  ot"  refrangibility. 

244.  When  light  is  made  to  pass  through  a  prism,  the 
different  colored  rays  are  separated,  and  form  an  image 
on  a  screen  or  wall,  in  which  the  colors  will  be  arranged 
in  the  order  just  mentioned. 

1.  Fig.  122  represents  rays  of  light  passing  from  the  aper- 
ture, in  a  window-shutter,  A  B,  through  the  prism  P.  Instead 


Fig.  122. 


Violet... 

Indig-o. . 

Bine  

Green . . 
Yellow. 
Orang'e. 
Red.... 


D  E 


of  continuing  in  a  straight  course  to  E,  and  there  forming  an 
image,  they  will  be  refracted,  in  their  passage  through  the 
prism,  and  form  an  image  on  the  screen,  C  D.  But  as  the 
different  colored  rays  have  different  degrees  of  refrangibility, 
those  which  are  i^efracted  the  least  will  fall  upon  the  lowest 
part  of  the  screen,  and  those  which  are  refracted  the  most  will 
fall  upon  the  highest  part.  The  red  rays,  therefore,  suffering 
the  smallest  degree  of  refraction,  fall  on  the  lowest  part  of  the 
screen,  and  the  remaining  colors  are  arranged  in  the  order  of 
their  refraction.'^ 

*  It  is  supposed  that  the  red  rays  are  refracted  the  least,  on  account  of 
their  greater  momentum,  and  that  the  blue,  indigo,  and  violet  are  refracted 
the  most,  because  they  have  the  least  momentum.  The  same  lenson,  it 
is  supposed,  will  account  for  the  red  appearance  of  the  sun,  through  a  fog, 
or  at  rising  and  setting.    The  increased  quantity  of  the  atmosphere,  which 


Do  these  rays  all  have  the  same  degree  of  refrangibility  ? 

244.  What  takes  place  when  light  is  made  to  pass  through  a  prism? 
Explain  Fig.  122.  Why  do  the  red  rays  fall  on  the  lowest  part  of  the 
screen?  What  is  supposed  with  regard  to  the  red  rays?  What  with 
regard  to  the  blue,  indigo,  and  violet  rays  ?  Why  does  the  sun  appear  red 
through  a  fog? 


188 


NATURAL  PHILOSOPHY. 


2.  If  the  colored  rays,  which  have  been  separated  by  a 
prism,  fall  upon  a  convex  lens,  they  will  converge  to  a  focus, 
and  appear  white.  Hence,  it  appears,  that  white  is  not  a 
simple  color,  but  is  produced  by  the  union  of  several  colors. 

3.  The  spectrum,  formed  by  a  glass  prism,  being  divided  in- 
to 360  parts,  it  is  found  that  the  red  occupies  45  of  those 
parts,  the  orange  27,  the  yellow  48,  the  green  60,  the  blue  60, 
the  indigo  40,  and  the  violet  80.^  By  mixing  the  seven 
primitive  colors  in  these  proportions,  a  white  is  obtained ;  but, 
on  account  of  the  impurity  of  all  colors,  it  will  be  of  a  dingy 
hue.  If  the  colors  were  more  clearly  and  accurately  defined, 
the  white,  thus  obtained,  would  appear  more  pure  also.  An 
experiment  to  prove  what  has  just  been  said  may  be  thus  per- 
formed. Take  a  circular  piece  of  board,  or  card,  and  divide  it 
into  parts,  by  hues  drawn  from  the  centre  to  the  circumference. 
Then,  having  painted  the  seven  colors  in  the  proportions  above 
named,  cause  the  board  to  revolve  rapidly  around  a  pin  or  wire 
at  the  centre.  The  board  will  then  appear  of  a  white  color. 
From  this,  it  is  inferred,  that  the  whiteness  of  the  sun's  light 
arises  from  a  due  mixture  of  all  the  primary  colors. 

4.  The  colors  of  all  bodies  are  either  the  simple  colors,  as 

the  oblique  rays  must  traverse,  and  its  being  loaded  with  mists  and  vapors, 
which  are  usually  formed  at  those  times,  prevents  the  other  rays  from 
reaching  us. 

A  similar  reason  will  account  for  the  blue  appearance  of  the  _sky.  As 
these  rays  have  less  momentum,  they  cannot  traverse  the  atmosphere  so 
readily  as  the  other  rays,  and  they  are,  therefore,  reflected  back  to  our 
eyes  by  the  atmosphere.  If  the  atmosphere  did  not  reflect  any  rays,  the 
skies  would  appear  perfectly  black. 

*  Light  is  found  to  possess  both  heat  and  chemical  action.  The  pris- 
matic spectrum  presents  some  remarkable  phenomena  with  regard  to  these 
qualities  ;  for  while  the  red  rays  appear  to  be  the  seat  of  the  maximum  of 
heat,  the  violet,  on  the  contrary,  are  the  apparent  seat  of  the  maximum 
of  chemical  action. 


Why  does  the  sky  appear  of  a  blue  color  ?  What  would  be  the  appear- 
ance of  the  sky  if  the  atmosphere  did  not  reflect  any  rays?  Is  white  a 
simple  color  ?  How  is  it  produced  ?  The  spectrum  formed  by  a  prism, 
being  divided  into  360  parts,  how  many  of  these  parts  does  the  red  occu- 
py? The  orange?  The  yellow?  The  green?  The  blue?  The  indigo? 
The  violet?    What  are  the  colors  of  all  bodies? 


opTrcs. 


189 


refracted  by  tlie  prism,  or  such  compound  colors  as  arise  from 
a  mixture  of  two  or  more  of  Lhem.* 

245.  The  rainbow  is  produced  by  the  refraction  of 
the  sun's  rays  in  their  passage  through  a  shower  of 
rain ;  each  drop  of  which  acts  as  a  prism  in  separating 
the  colored  rays,  as  they  pass  through  it. 

This  is  proved  by  the  following  considerations.  First,  A 
rainbow  is  never  seen  except  when  rain  is  falling,  and  the  sun 
shining  at  the  same  time ;  and  that  the  sun  and  the  bow  are 
always  in  opposite  parts  of  the  heavens  ;  and,  secondly,  that 
the  same  appearance  may  be  produced  artificially,  by  means  of 
water  thrown  into  the  air,  when  the  spectator  is  placed  in  a 
proper  position,  with  his  back  to  the  sun ;  and,  thirdly,  that  a 
similar  bow  is  generally  produced  by  the  spray  which  arises 
from  large  cataracts,  or  waterfalls. f 

246.  The  color  of  all  bodies  depends  upon  the  rays 
which  they  reflect. 

1,  Some  bodies  absorb  all  the  rays  which  they  receive  ex- 
cept the  red  rays.  These  bodies,  therefore,  appear  of  a  red 
color ;  some  reflect  the  green,  and  absorb  all  the  others, — 
these  will  appear  of  a  green  color ;  and,  in  general,  bodies  ap- 
pear of  the  col^  of  those  rays  which  they  reflect,  while  they 
absorb  all  the  other  rays.  Sometimes  a  body  reflects  a  portion 
of  the  rays  of  several  colors.  The  body  will  then  appear  of  a 
compound  color,  composed  of  the  various  colors  which  it  re- 
flects. When  a  body  reflects  all  the  rays,  it  appears  white, — 
when  it  absorbs  all  the  rays,  it  appears  black.    White,  then, 

*  From  the  experiments  of  Dr.  Wollaston,  it  appears  that  the  seven 
colors  formed  by  the  prism  may  be  reduced  to  four,  namely,  red,  green, 
blue,  and  violet ;  and  that  the  other  colors  are  produced  by  combinations 
of  these. 

t  The  Falls  of  Niagara  afford  a  beautiful  exemplification  of  the  truth 
of  this  observation. 


Note.  What  appears  from  the  experiments  of  Dr.  Wollaston  ? 

245.  How  is  the  rainbow  produced?  How  is  this  proved?  First? 
Second?  Third? 

246.  Upon  what  does  the  color  of  all  bodies  depend?  Of  what  color  do 
bodies  generally  appear?  When  will  a  body  appear  of  a  compound  color? 
Of  what  color  will  a  body  appear  that  reflects  all  the  rays?  When  will  a 
body  appear  black  ? 


190 


NATURAL  PHILOSOPHY. 


is  a  mixture  of  all  the  primitive  colors,  and  black  is  the  depn- 
vation  of  all  color. 

2.  From  what  has  now  been  said,  it  appears,  that  no  body 
has  a  permanent  or  intrinsic  color  of  its  own, — but  that  color, 
as  well  as  weight,  are  accidental^  and  not  essential  properties. 
All  substances  appear  of  the  same  color,  or  rather,  more 
properly  speaking,  are  deprived  of  all  color,  in  the  dark. 

3.  Light,  from  whatever  source  it  proceeds,  is  of  the  same 
nature,  composed  of  the  various  colored  rays ;  and  although 
some  substances  appear  differently  by  candlelight,  from  what 
they  appear  by  day,  this  result  may  be  supposed  to  arise  from 
the  weakness  or  want  of  purity  in  artificiid  light. 

4.  There  can  he  no  light  without  colors,  and  there  can  he  no 
colors  without  light. 

5.  That  the  above  remarks,  in  relation  to  the  colors  of  bodies, 
are  true,  may  be  proved  by  the  following  simple  experiment. 
Place  a  colored  body  in  a  dark  room,  in  a  ray  of  light  that  has 
been  refracted  by  a  prism ;  the  body,  of  whatever  color  it 
naturally  is,  will  appear  of  the  color  of  the  ray  in  which  it  is 
placed  ;  for,  since  it  receives  no  other  colored  rays,  it  can  reflect 
no  others."^ 

*  Although  bodies,  from  the  arrangement  of  their  f^rticles,  have  a  ten- 
dency to  absorb  some  rays,  and  reflect  others,  they  are  not  so  uniform  in 
their  arrangement  as  to  reflect  only  pure  rays  of  one  color,  and  perfectly 
absorb  all  others  ;  it  is  found,  on  the  contrary,  that  a  body  reflects  in  great 
abundance  the  rays  which  determine  its  color,  and  the  others  in  a  greater 
or  less  degree,  in  proportion  as  they  are  nearer  or  further  from  its  color,  in 
the  order  of  refrangibility.  Thus,  the  green  leaves  of  a  rose  will  reflect  a 
few  of  the  red  rays,  which  will  give  them  a  brown  tinge.  Deepness  of 
color  proceeds  from  a  deficiency  rather  than  -from  an  abundance  of  re- 
flected rays.  Thus,  if  a  body  reflect  only  a  few  of  the  green  rays,  it  will 
appear  of  a  dark  green.  The  brightness  and  intensity  of  a  color  shows 
that  a  great  quantity  of  rays  are  reflected.  That  bodies  sometimes  change 
their  color,  is  owing  to  some  chemical  change,  which  takes  place  in  the 


Is  color  an  essential  property  of  a  body  ?  Of  what  color  do  bodies  ap- 
pear in  the  dark?  Why  do  some  bodies  appear  differently  by  candlelight? 
What  is  necessary  to  produce  color?  What  experiments  are  related  to 
prove  the  truth  of  the  above  ?  Note.  What  rays  does  a  body  reflect  in  the 
greatest  abundance?  In  what  proportion  does  it  reflect  the  other  rays? 
Why  do  the  green  leaves  of  a  rose  appear  to  have  a  brown  tinge  ?  What 
does  the  brightness  and  intensity  of  a  color  show  ?  Why  do  some  bodies 
change  their  color? 


OPTICS. 


191 


247.  A  multiplying-glass  is  a  convex  lens,  one  side 
of  which  is  ground  down  into  several  flat  surfaces. 

When  an  object  is  viewed  through  a  multiplying-glass,  it 
will  be  multiplied  as  many  times  as  there  are  flat  surfaces  on 
the  lens.  Thus,  if  one  lighted  candle  be  viewed  through  a  lens, 
having  twelve  flat  surfaces,  twelve  candles  will  be  seen  through 
the  lens.  The  principle  of  the  multiplying-glass  is  the  same 
with  that  of  a  convex  or  concave  lens. 

248.  The  Kaleidescope*  consists  of  two  reflecting 
surfaces,  or  pieces  of  looking-glass,  inclined  to  each 
other  at  an  angle  of  60  degrees,  and  placed  between  the 
eye  and  the  objects  intended  to  form  the  picture. 

The  two  plates  are  enclosed  in  a  tin  or  paper  tube,  and  the 
objects,  consisting  of  pieces  of  colored  glass,  beads,  or  other 
highly-colored  fragments,  are  loosely  confined  between  two 
circular  pieces  of  common  glass,  the  outer  one  of  which  is 
slightly  ground,  to  make  the  light  uniform.  On  looking  down 
the  tube  through  a  small  aperture,  and  where  the  ends  of  the 
glass  plates  nearly  meet,  a  beautiful  figure  will  be  seen,  having 
six  angles,  the  reflectors  being  inclined  the  sixth  part  of  a  cir- 
cle, if  inclined  the  twelfth  part,  or  twentieth  part  of  a  circle, 
twelve  or  twenty  angles  will  be  seen.  By  turning  the  tube  so 
as  to  alter  the  position  of  the  colored  fragments  within,  these 
beautiful  forms  will  be  changed ;  and  in  this  manner  an  almost 
infinite  variety  of  patterns  may  be  produced. 

OF  THE  THERMAL,  CHEMICAL,  AND  OTHER  NON-OPTICAL 
EFFECTS  OF  LIGHT. 

1.  The  science  of  Optics  treats  particularly  of  light  as  the 
medium  of  vision.    But  there  are  other  efl'ects  of  this  agent, 

internal  arrangement  of  their  parts,  whereby  they  lose  their  tendency  to 
reflect  certain  colors,  and  acquire  the  power  of  reflecting  others. 

*  The  word  Kaleidescope  is  derived  from  the  Greek  language,  and 
means,  "  the  sight  of  a  beautiful  form."  The  instrument  was  invented  by 
Dr.  Brewster,  of  Edinburgh,  a  few  years  ago. 

247.  What  is  a  multiplying-glass?  How  many  times  will  an  object, 
viewed  through  a  multiplying-glass,  be  multiplied?  What  is  the  principle 
of  the  multiplying-glass? 

248.  Of  what  does  the  kaleidescope  consist  ?  Note.  From  what  is  the 
word  kaleidescope  derived,  and  what  does  it  mean?  By  whom  was  the 
mstrument  invented  ?    What  is  here  said  with  regard  to  the  kaleidescope  ? 


4 


192 


NATURAL  PIIILOSOPHY. 


which,  although  more  immediately  connected  with  the  science 
of  Chemistry,  deserve  to  be  noticed  in  this  connexion. 

2.  The  thermal  effects  of  light,  that  is,  its  agency  in  the 
excitation  of  heat,  when  it  proceeds  directly  from  the  sun, 
are  well  known.  But  it  is  not  generally  known  that  these 
effects  are  extremely  unequal  in  the  differently  colored  rays,  as 
they  are  refracted  by  the  prism.  It  has  already  been  stated 
in  a  note  on  page  186,  that  the  red  rays  appear  to  possess  the 
thermal  properties  in  the  greatest  degree,  and  that  in  the  other 
rays  in  the  spectrum  there  is  a  decrease  of  thennal  power 
towards  the  violet,  where  it  ceases  altogether.  But,  on  the 
contrary,  that  the  chemical  agency  is  the  most  powerful  in  the 
violet,  from  which  it  constantly  decreases  towards  the  red, 
where  it  ceases  altoofether.  Whether  these  thermal  and  chemi- 
cal  powers  exist  in  all  light,  from  whatever  source  it  is  derived, 
remains  yet  to  be  ascertained.  The  chromatic  intensity  of  the 
colored  spectrum  is  greatest  in  the  yellow,  from  whence  it  de- 
creases both  ways,  terminating  almost  abruptly  in  the  red,  and 
decreasing  by  almost  imperceptible  shades  towards  the  violet, 
where  it  becomes  faint,  and  then  wholly  indistinct.  Thus  it 
appears  that  the  greatest  heating  power  resides  where  the 
chemical  power  is  feeblest,  and  the  greatest  chemical  power 
where  the  heating  power  is  feeblest,  and  that  the  optical  power 
is  the  strongest  between  the  other  two. 

3.  The  chemical  properties  of  light  are  shown  in  this,  that 
the  light  of  the  sun,  and  in  an  inferior  degree  that  of  day 
when  the  sun  is  hidden  from  view,  is  a  means  of  accelerating 
chemical  combinations  and  decompositions.  The  following  ex- 
periment exhibits  the  chemical  effects  of  light  : 

Place  a  mixture  of  equal  parts  (by  measiu'e)  of  chlorine  and 
hydrogen  gas  in  a  glass  vessel,  and  no  change  will  happen  so 
long  as  the  vessel  be  kept  in  the  dark  and  at  an  ordinary  tem- 
perature ;  but  on  exposing  it  to  the  dayhght,  the  elements  will 
slowly  combine  and  form  hydrochloric  acid  ;  if  the  glass  be  set 


Iq  what  way  does  Optics  treat  of  light  ?  What  is  the  thermal  property 
of  lifht?  Are  the  thermal  properties  of  the  rays  the  same,  or  different  in 
each  color  of  the  prism  ?  How  do  they  differ  ?  How  do  the  chemical 
•agencies  differ  ?  Do  these  powers  exist  in  all  light,  or  in  solar  light  only  ? 
Where  is  the  greatest  heating  power  found  ?  Where  the  greatest  chemi- 
cal power?    Where  the  greatest  optical  power? 

In  what  are  the  chemical  powers  of  light  shown?  Name  the  experi- 
ments and  explain  them. 

4 


OPTICS 


193 


in  the  sun's  niys,  the  union  will  be  accompanied  with  an  in- 
st:intaneous  detonation.  The  report  may  also  be  produced 
by  transmittifig'  ordinary  daylight  througli  violet  or  blue  glass 
to  the  mixture,  but  by  interposing  a  red  glass  between  the  ves- 
sel and  the  light,  all  combination  of  the  elements  is  prevented. 

4.  The  chemical  effects  of  light  have  recently  been  employed 
to  render  permanent  the  images  obtained  by  means  of  convex 
lenses.  The  art  of  thus  fixing  them  is  termed  Photography  or 
Hehography.^  The  mode  in  which  the  process  is  performed 
is  essentially  as  follows : — The  picture,  formed  by  a  camera 
obscura,  is  received  on  a  plate,  the  surface  of  which  has  been 
previously  prepared,  so  as  to  make  it  as  susceptible  as  possible 
of  the  chemical  influence  of  light.  After  the  lapse  of  a  longer 
or  shorter  time,  the  light  will  have  so  acted  on  the  plate  that 
the  various  objects,  the  images  of  which  were  projected  upon 
it,  will  appear,  with  all  their  gradations  of  light  and  shade, 
most  exactly  depicted  in  black  and  white,  no  color  being- 
present.  This  is  the  process  commonly  known  by  the  name  of 
Daguerreotype,  from  M.  Daguerre,  the  author  of  the  discovery. 
Since  his  original  discovery  he  has  ascertained  that  by  isolating 
and  electrifying  the  plate,  it  acquired  such  a  sensibility  to  the 
chemical  influence  of  light,  that  one-tenth  of  a  second  is  a  suf- 
ficient time  to  obtain  the  requisite  luminous  impression  for  the 
formation  of  the  picture. 

5.  The  chemical  efl'ects  of  light  are  seen  in  the  varied  colors 
of  the  vegetable  world.  Vegetables  grown  in  dark  places  are 
either  white  or  of  a  palish  yellow.  The  sunny  side  of  fruics  is 
of  a  richer  tinge  than  that  which  grows  in  the  shade.  Persons 
whose  daily  emploj^ment  keeps  them  much  within  doors,  are 
pale,  and  more  or  less  sickly  in  consequence  of  such  confine- 
ment. 

From  what  has  now  been  detailed  with  regard  to  the  nature, 
the  efl'ects,  and  the  importance  of  light,  we  may  see  with  what 
reason  the  great  epic  poet  of  our  language  has  apostrophized 
tt  in  the  words  : 

"  Hail,  holy  Light !  offspring  of  Heaven,  first-born, 
Bright  efiluence  of  bright  essence  increate," 

*  These  words  are  Greek  derivatives  ;  the  former  meaning  "  writing  oi 
drawing  by  means  of  light"  the  latter,  "  writing  or  drawing  by  the  aid 
of  the  sun." 

Explain  the  art  of  Piiotography,  or  Heliography.    By  what  name  is  h 
now  known  ?    How  else  are  the  chemical  effects  of  light  seen? 
9 


194 


NATURAL  PHILOSOPHY. 


and  why  the  author  of  the  Seasons  has  in  a  similar  manner 
addressed  it  in  the  terms : 

"  Prime  cheerer,  Light ! 
Of  all  material  beings  first  and  best  I 
Efflux  divine  !  Nature's  resplendent  robe  ! 
Without  whose  vesting  beauty  all  were  wrapt 
In  unessential  gloom  ;  and  thou,  O  Sun ! 
Soul  of  surrounding  worlds,  in  w^hom  best  seen 
Shines  out  thy  Maker  !  may  I  sing  of  thee  ?" 


CHAPTER  XL 

ELECTRICITY. 

249.  The  word  Electricity*  is  a  term  used  by  phi- 
losophers to  signify  the  operations  of  a  very  subtle  and 
elastic  fluid,  which  pervades  the  material  world.  Elec- 
tricity can  be  seen  only  in  its  effects;  which  are  ex- 
hibited in  the  form  of  attraction  and  repulsion. 

If  a  piece  of  amber,  sealing-wax,  or  smooth  glass,  perfectly 
clean  and  dry,  be  briskly  rubbed  with  a  dry  woollen  cloth,  and 
immediately  afterwards  held  over  small  and  light  bodies,  such 
as  pieces  of  paper,  thread,  cork,  straw,  feathers,  or  fragments 
of  gold  leaf,  strewed  upon  a  table,  these  bodies  will  be  attract- 
ed, and  fly  towards  the  surface  that  has  been  rubbed,  and  ad- 
here to  it  for  a  certain  time.  The  surfaces  that  have  acquired 
this  power  of  attraction  are  said  to  be  excited ;  and  the  sub- 
stances thus  susceptible  of  being  excited  are  called  electrics, 
while  those  which  cannot  be  excited  in  a  similar  manner  are 
called  non-electrics. 

*  This  word  is  derived  from  a  Greek  word,  which  signifies  amber,  be- 
cause this  substance  was  supposed  to  possess,  in  a  remarkable  degree,  the 
property  of  producing  the  fluid,  when  excited  or  rubbed.  The  property  it- 
self was  first  discovered  by  Thales  of  Miletus,  one  of  the  seven  wise  men 


249.  What  is  electricity  ?  How  can  electricity  be  seen  ?  How  are 
these  effects  exhibited?  What  illustration  of  this  is  given  ?  What  is  said 
rf  the  surfaces  which  have  acquired  the  power  of  attraction?  What  are 
electrics  ?  What  are  non-electrics  ?  Note.  What  is  stated  with  regard 
to  the  word  electricity  ?    By  whom  was  this  property  first  discovered  ? 


EriKCTlUClTY. 


195 


250.  The  science  of  electricity,  therefore,  divides  all 
substances  into  two  kinds  ;  namely.  Electrics,  or  those 
substances  which  can  be  excited,  and  Non-electrics,  or 
those  substances  which  cannot  be  excited. 

of  Greece.  The  word  is  now  used  to  express  both  the  fluid  itself,  and  the 
science  which  treats  of  it. 

The  nature  of  electricity  is  entirely  unknown.  Some  philosophers  con- 
sider it  a  fluid  ;  others  consider  it  as  two  fluids  of  opposite  qualities ;  and 
others  again  deny  its  materiality,  and  deem  it,  hke  attraction,  a  mere  prop- 
erty of  matter.  In  this  volume  the  opinion  of  Dr.  Franklin  is  adopted, 
who  supposed  it  to  be  a  single  fluid,  disposed  to  difllise  itself  equally  among 
all  substances,  and  exhibiting  its  peculiar  effects  only  when  a  body  by  any 
means  becomes  possessed  of  more  or  less  than  its  proper  share.  That  when 
any  substance  has  more  than  its  natural  share,  it  is  said  to  be  positively 
electrified,  and  that  when  it  has  less  than  its  natural  share,  it  is  said  to  be 
negatively  electrified ;  that  positive  electricity  implies  a  redundancy,  and 
negative  electricity  a  deficiency  of  the  fluid.  The  adoption  of  this  opinion 
of  Franklin  is  a  matter  of  mere  verbal  preference  ;  for  whether  the  effects 
described  under  the  name  of  electricity  be  the  effects  of  one  fluid  moving 
in  a  particular  direction,  or  of  two  fluids  moving  in  opposite  directions, 
or  no  motion  of  a  fluid  at  all,  the  facts  remain  the  same  whatever  the 
theory  may  be,  and  they  may  be  explained  as  well  under  one  set  of 
terms  as  another.  Professor  Faraday  has  proposed  a  nomenclature  of  elec- 
tricity, which  has  been  adopted  in  some  scientific  treatises.  From  the 
Greek  words  ^XcKvpov,  (electricity,  or  amber,  from  which  it  was  first  pro- 
duced,) and  Uog,  (a  way  or  path,)  he  formed  the  word  electrodes,  that  is, 
ways  or  paths  of  electricity.  The  course  of  positive  electricity  he  called 
the  anode,  (from  the  Greek  avohog,  an  ascending  or  entering  way,)  and  the 
course  of  the  negative  electricity  the  cathode,  (from  the  Greek  Kado6os,  a 
descending  way,  or  path  of  exit.)  The  terms  positive  and  negative  are, 
however,  more  frequently  employed  to  designate  the  extremities  of  the 
channels  through  which  electricity  passes.  Positive  electricity  is  some- 
times expressed  by  the  term  plus,  or  its  character  -{-,  and  negative  elec- 
tricity by  the  term  minus,  or  its  character  — . 

Electricity  may  be  excited  by  several  modes — as  1st,  by  friction,  whence 

What  is  stated  with  regard  to  the  nature  of  electricity  ?  Whose  and 
what  opinion  is  adopted  in  this  volume  ?  When  is  a  substance  said  to  be 
positively  electrified  ?  When  is  it  said  to  be  negatively  electrified  ?  What 
does  positive  electricity  imply  ?  What  does  negative  electricity  imply  ? 
In  how  many  ways  may  electricity  be  excited  ?  What  are  the  different 
kinds  of  electricity  called? 

250.  Into  how  many  kinds  does  the  science  of  electricity  divide  all  sub- 
stances ?    What  are  they  ? 


196 


NATURAL  rillLOSOPnY. 


251.  The  electric  fluid  is  readily  communicated  from 
one  substance  to  another.  Some  substances,  however, 
will  not  allow  it  to  pass  through  or  over  them,  while  others 
sive  it  a  free  passage.  Those  substances  th  1 0Uj_j  h  w  hich  it 
passes  without  obstruction  are  called  eonductors;  while 
those  through  w^iich  it  cannot  readily  pass  are  called 
non-conductors  ;  and  it  is  found,  by  experiment,  that  all 
electrics  are  non-conductors,  and  all  non-electrics  are 
good  conductors  of  electricity. 

1.  The  following  substances  are  electrics,  or  non-conductora 
of  electricity ;  namely, 

Atmospheric  air,  (when  dry,)  Feathers, 
Glass,  Amber, 
Diamond,  Sulphur, 
All  precious  stones,  Silk, 
All  gums  and  resins.  Wool, 
The  oxides  of  all  metals,  Hair, 
Beeswax,  Paper, 
Sealing-wax,  Cotton. 
All  these  substances  must  be  dry,  or  they  will  become  more 
or  less  conductors. 

2.  The  following  substances  are  non-electrics,  or  conductors 
of  electricity  ;  namely. 

All  metals,  Living  ammals. 

Charcoal,  ^^apor,  or  steam, 

it  is  called  Frictional  Electricity;  2dly,  hy  chemical  action,  called,  from 
its  discoverers,  Galvanic  or  Voltaic  Electricity;  3dly,  by  the  action  of 
heat,  whence  it  is  called  Thermo-Electricity ;  4thly,  hy  Magnetism. 
Frictional  Electricity  forms  the  subject  of  that  branch  of  Electricity  usually 
treated  under  the  head  of  Natural  Philosophy.  Electricity,  excited  by 
chemical  action,  forms  the  subject  of  Galvanism;  and  Electricity  produced 
by  the  agency  of  heat,  or  by  :\Iagnetism,  is  usually  considered  in  connexion 
with  the'subject  of  Electro-Magnetism.  The  intimate  connexion  between 
these  several  subjects  shows  how  close  are  the  links  of  the  chain  by  which 
all  the  departments  of  physical  science  are  united 

251.  What  is  said  with  regard  to  the  communication  of  the  electric  fluid 
from  one  substance  to  another?  Will  all  substances  allow  it  to  pass  through 
them?  What  bodies  are  called  conductors?  What  bodies  are  called  non 
conductors?  What  has  been  found,  by  experiment,  with  regard  to  elec 
tries  and  non-electrics  ?  What  substances  are  electrics  or  non-conductors  ? 
Why  must  these  substances  be  dry?  What  substances  are  non-electrics 
or  conductors? 


ELECTRICITY. 


197 


S.  Tlio  following  are  imperfect  conductors,  (that  is,  they 
conduct  the  electric  lluid,  but  not  so  readily  as  the  substances 
above  mentioned,)  namely. 

Water,  Common  wood, 

Green  vegetables,  Dead  animals, 

Damp  air,  Bone, 
Wet  wood,  Horn,  &c. 

All  substances  containing  moisture, 

252.  When  a  conductor  is  surrounded  on  all  sides  by 
non-conducting  substances,  it  is  said  to  be  insulated, 

1.  As  glass  is  a  non-conducting  substance,  any  conducting 
substance  surrounded  with  glass,  or  standing  on  a  table  or 
stool,  with  glass  legs,  will  be  insulated. 

2.  As  the  air  is  a  non-conductor,  when  dry,  a  substance 
which  rests  on  any  non-conducting  substance  will  be  insulated, 
unless  it  communicate  with  the  ground,  the  floor,  a  table,  &c. 

253.  When  a  communication  is  made  between  a  con- 
ductor and  an  excited  surface,  the  electricity  from  the 
excited  surface  is  immediately  conveyed  by  the  con- 
ductor to  the  ground  but  if  the  conductor  be  insulated, 
its  whole  surface  will  become  electrified,  and  it  is  said 
to  be  charged. 

*  The  earth  may  be  considered  as  the  principal  reservoir  of  electricity ; 
and  v^hen  a  communication  exists,  by  means  of  any  conducting  substance, 
between  a  body  containing  more  than  its  natural  share  of  the  fluid,  and 
the  earth,  the  body  will  immediately  lose  its  redundant  quantity,  and  the 
fluid  will  escape  to  the  earth.  Thus,  when  a  person  holds  a  metallic  tube 
to  an  excited  surface,  the  electricity  escapes  from  the  surface  to  the  tube, 
and  passes  from  the  tube  through  the  person  to  the  floor ;  and  the  floor 
being  connected  with  the  earth  by  conducting  substances,  such  as  the 
timbers,  &c.,  which  support  the  building,  the  electricity  will  finally  pass  off 
by  a  regular  succession  of  conducting  substances,  from  the  excited  surface 
to  the  earth.    But  if  the  chain  of  conducting  substances  be  interrupted, 


What  substances  are  mentioned  as  imperfect  conductors  ? 

252.  When  is  a  substance  said  to  be  insulated?    Give  the  examples. 

253.  When  a  communication  is  made  between  a  conductor  and  an  ex- 
cited sarfaqe,  where  is  the  electricity  from  the  excited  substance  conveyed  ? 
When  is  it  said  to  be  charged?  I^ote.  When  a  communication  exists  by~ 
means  of  any  conducting  substance,  between  a  body  containing  more  than 
its  natural  share  of  the  fluid  and  the  earth,  what  will  become  of  the  re- 
dnudant  quantity  which  the  body  possesses  ? 


198 


NATURAL  PHILOSOPHY. 


254.  The  simplest  mode  of  exciting  electricity  is  by 
friction. 

Thus,  if  a  thick  cyhnder  of  sealing-wax,  or  sulphur,  or  a 
glass  tube,*  be  rubbed  with  a  silk  handkerchief,  a  piece  of 
clean  flannel,  or  the  fur  of  a  quadruped,  the  electric  fluid  will 
be  excited,  and  may  be  communicated  to  other  substances  from 
the  electric  thus  excited. 

255.  The  electricity  excited  in  glass  is  called  the 
vitreous  or  positive  electricity ;  and  that  obtained  from 
sealing-wax,  or  other  resinous  substances,  is  called 
resinous  or  negative  electricity. 

256.  The  vitreous  and  resinous,  or,  in  other  words, 
the  positive  and  negative  electricities,  always  accom- 
pany each  other  ;  for  if  any  surface  become  positive,  the 
surface  with  which  it  is  rubbed  will  become  negative  ; 
and  if  any  surface  be  made  positive,  the  nearest  con- 
ducting surface  will  become  negative.  And  if  positive 
electricity  be  communicated  to  one  side  of  an  electric, 
(as  a  pane  of  glass,  or  a  glass  vial.)  the  opposite  side 
will  become  negatively  electrified,  and  the  plate  or  the 
glass  is  then  said  to  be  charged. 

1.  When  one  side  of  a  metallic,  or  other  conductor,  receives 

that  is,  if  anv  non-conducting  substance  occur  bet\;veen  the  excited  surface 
and  the  course  which  the  fluid  takes  in  its  progress  to  the  earth,  the  con- 
ducting substances  will  be  insulated,  and  become  charged  with  electricity. 
Thus,  if  an  excited  surface  be  connected  by  a  long  chain  to  a  metallic  tube, 
and  the  metaUic  tube  be  held  by  a  person  who  is  standing  on  a  stool  with 
glass  legs,  or  on  a  cake  of  sealing-wax,  resin,  or  any  other  non-conducting 
substance,  the  electricity  cannot  pass  to  the  ground,  and  the  person,  the 
chain,  and  the  tube  will  all  become  electrified. 

*  Whatever  substance  is  used,  it  must  be  perfectly  drs\  If,  therefore, 
a  glass  tube  be  used,  it  should  previously  be  held  to  the  fire,  and  gently 
warmed,  in  order  to  remove  all  moisture  from  its  surface. 


^Vhat  illustration  of  this  is  given  ?  What  follows  if  this  chain  of  con- 
ducting substances  be  interrupted? 

254.  What  is  the  simplest  mode  of  exciting  electricity  ?  What  illustra- 
tion of  this  is  given  ? 

255.  What  is  the  electricity  excited  in  glass  called  ?  What  is  that  ob- 
tained from  resinous  substances  called  ? 

256  What  is  stated  with  regard  to  positive  and  negative  electricity  ? 


ELECTRICITY. 


199 


the  electric  fluid,  its  wliole  sui  face  is  instantly  pervaded ;  but 
Avhen  an  electric  is  presented  to  an  electrified  body,  it  becomes 
electrified  in  a  small  spot  only. 

2.  When  two  surfaces  oppositely  electrified  are  united,  their 
powers  are  destroyed  ;  and  if  their  union  be  made  through  the 
human  body,  it  produces  an  affection  of  the  nerves,  called  an 
electric  shock. 

257.  Similar  states  of  electricity  repel  each  other ; 
and  dissimilar  states  attract  each  other. 

Thus,  if  two  pith-balls  suspended  by  a  silk  thread,  are  both 
positively  or  both  negatively  electrified,  they  will  repel  each 
other ;  but  if  one  tie  positively  and  the  other  negatively  elec- 
trified, they  will  attract  each  other. 

258.  The  Leyden  jar  is  a  glass  vessel  used  for  the 
purpose  of  accumulating  the  electric  fluid,  procured 
from  excited  surfaces. 

1.  Fig.  123  represents  a  Leyden  jar.  It  is  a  glass  jar, 
coated  both  on  the  inside  and  the  outside  with  tinfoil,  with 
a  cork  or  wooden  stopper  through  which  a 
metallic  rod  passes,  terminating  upwards  in  a 
brass  knob,  and  connected  by  means  of  a  wire, 
at  the  other  end,  with  the  inside  coating  of  the 
jar.  The  coating  extends  both  on  the  inside 
and  outside  only  to  within  two  or  three  inches 
of  the  top  of  the  jar.  Thus  prepared,  when  an 
excited  surface  is  applied  to  the  brass  knob,  or 
connected  with  it  by  any  conducting  surface,  it 
parts  with  its  electricity,  the  fluid  enters  the  jar, 
and  the  jar  is  said  to  be  charged. 

When  the  Leyden  jar  is  charged,  the  fluid  is 
contained  in  the  inside  coating  of  the  vial ;  and 
as  this  coating  is  insulated,  the  fluid  will  remain 
in  the  jar  until  a  communication  be  made,  by 


What  follows  when  one  side  of  a  metallic,  or  other  conductor,  receives 
the  electric  fluid?  What  follows  when  an  electric  is  presented  to  an  elec- 
trified body  ?  What  follows  when  two  surfaces,  oppositely  electrified,  are 
united  ? 

257.  How  do  similar  states  of  electricity  act  on  each  other?  How  do 
dissimilar  states  act  on  each  other?    Give  the  examples. 

258.  For  what  is  the  Leyden  jar  used?  What  does  Fig.  123  represent? 
What  is  a  Leyden  jar?    When  is  the  Leyden  jar  said  to  be  charged? 


200 


NATURAL  PHILOSOPHY. 


means  of  some  conducting  substance,  between  the  inside  and 
the  outside  of  the  jar.  If  then  a  person  apply  one  hand  or 
finger  to  the  brass  knob,  and  the  other  to  the  outside  coating 
of  the  jar,  a  communication  will  be  formed  by  means  of  the 
brass  knob  with  the  inside  and  outside  of  the  jar,  and  the  jai- 
w411  be  discharged.  A  vial  or  jar  that  is  insulated  cannot  he 
charged. 

259.  An  electrical  battery  is  composed  of  a  number 
of  Leyden  jars  connected  together. 

The  inner  coatings  of  the  jars  are  connected  together  bv 
chains  or  metallic  bars  attached  to  the  brass  knobs  of  each 
jar  ;  and  the  outer  coatings  have  a  similar  connexion  establish- 
ed by  placing  the  vials  on  a  sheet  of  tinfoil.  The  wdiole  bat- 
tery may  then  be  charged  like  a  single  jar.  For  the  sake  of 
convenience  in  discharging  the  battery,  a  knob,  connected  with 
the  tinfoil  on  which  the  jars  stand,  projects  from  the  bottom 
of  the  box  which  contains  the  jars. 

260.  The  jointed  discharger  is  an  instrument  used  to 
discharge  a  jar,  or  battery. 

Fig.  124  represents  the  jointed  discharger.    It  consists  of 
two  rods,  generally  of  brass,  terminating  at  one  end  in  brass 
balls,  and  connected  together  at  the 
other  end  by  a  joint,  like  that  of  a  ^ig.  124. 

pair  of  tongs,  allowing  them  to  be 
opened  or  closed.  It  is  furnished 
with  a  glass  handle,  to  secure  the 
person  who  holds  it  from  the  effects  of 
a  shock.  When  opened,  one  of  the 
balls  is  made  to  touch  the  outside 
coating  of  the  jar,  or  the  knob  con- 
nected with  the  bottom  of  the  battery,  and  the  other  is  applied 
to  the  knob  of  the  jar,  or  jars.  A  communication  being  thus 
formed  between  the  inside  and  the  outside  of  the  jar,  a  dis- 
charge of  the  fluid  will  be  produced. 


How  can  the  jar  be  discharged  ?    Can  an  insulated  jar  be  charged  ? 

259.  Of  what  is  an  electrical  battery  composed  ?  How  are  the  inner 
coatings  of  the  jars  connected  together  ?  How  are  the  outer  coatings 
connected?    In  what  way  is  tlie  battery  charged? 

260.  What  is  the  jointed  discharger  ?  What  does  Fig.  124  repi-pi-ent  ? 
Of  what  docs  it  consist  ? 


i:i,i:("i'iii(;rrv. 


201 


*J(U.  W'luMi  ;i  clinr'j:!'  of  cIcm-I  I'ic/ily  is  lo  Ix*.  sent 
throiiuli  ;iiiy  it iciila r  siil)sl;ni(*(\  the  suhslaiK'c  must 
torm  a  |)ail  of  circuit  of  the  elect ricit ij-^  that  is,  it 
must  he  j)la('(Ml  ill  siK-li  a  maimc.r  that  the  fhiid  (-aniiot 
pass  from  the  inside  to  tlie  outside  surla,(*e  of  the  jar,  or 
battery,  without  passiiiij^  throui^di  the  suhstaiiee  in  its 
passai!:e. 

MetalHc  rods,  witli  sharp  points,  silently  attract 
the  electric  Ihiid. 

1.  If  tlie  balls  bo  removed  from  the  jointed  discliar^^er,  and 
tlu^  two  rods  terminate  in  sliarp  points,  the  electiicily  will  pass 
oif  silently  and  produce  but  little  effect. 

2.  A  Leyden  jar,  or  a  battery,  may  be  silently  discharged  by 
presenting  a  metallic  point,  even  that  of  the  finest  needle,  to 
the  knob.  It  is  on  this  principle  that  lightning-rods  are  con- 
structed. The  electric  fluid  is  silently  drawn  from  the  cloud 
by  the  sharp  points  on  the  rods,  and  is  thus  prevented  from 
suddenly  exploding  on  high  buildings. 

o.  Electricity,  of  one  kind  or  the  other,  is  generally  induced 
in  sui-rounding  bodies  by  the  vicinity  of  a  highly-excited  elec- 
tric. This  mode  of  communicating  electricity  by  ap2^roach,  is 
styled  induction. 

4.  A  body,  on  approaching  another  body  powerfully  electri- 
ilv'd,  will  be  throw^n  into  a  contrary  state  of  electricity.  Thus, 
a  feather,  brought  near  to  a  glass  tube  excited  by  friction,  will 
be  attracted  by  it ;  and,  therefore,  previously  to  its  touching 
the  tube,  negative  electiicity  must  have  been  induced  in  it. 
On  the  contrary,  if  a  feather  be  brought  near  to  excited 
seoliiuj-icax,  it  will  be  attracted,  and,  consequently,  posiiive 
electricity  must  have  been  induced  in  it  before  contact. 

263.  When  electricity  is  comnianicated  from  one 
body  to  another  in  contact  with  it,  it  is  called  electricity 
hy  transfer, 

264.  The  electrical  machine  is  a  machine  constructed 

261.  What  is  necessary  when  a  charge  of  electricity  is  to  be  sent  through 
any  particular  substance  ? 

262.  In  what  way  do  metallic  rods,  with  sharp  points,  attract  the  elec- 
tric fluid?  How  can  the  electricity  be  made  to  pass  off  silently?  Upon 
what  principle  are  lightning-rods  constructed?  When  is  electricity  said 
to  be  communicated  by  induction? 

263.  When  is  electricity  produced  by  transfer? 

9* 


202 


NATURAL  PHILOSOPHY. 


for  the  purpose  of  accumulating  or  collecting  electricity, 
and  transferring  it  to  other  substances. 

1.  Electrical  machines  are  made  in  various  forms,  but  all  on 
the  same  principle,  namely,  the  attraction  of  metallic  points. 
The  electricity  is  excited  by  the  friction  of  silk  on  a  glass  sur- 
face, assisted  by  a  mixture  or  preparation  called  an  amalgam  * 
The  glass  surface  is  made  either  in  the  form  of  a  cylinder  or  a 
circular  plate,  and  the  machine  is  called  a  cylinder  or  a  plate 
machine,  according  as  it  is  made  with  a  cylinder  or  with  a  plate. 

2.  Fig.  125  represents  a  plate  electrical  machine.  A  D  is 
the  stand  of  the  mdchine,  L  L  L  L  are  the  four  glass  legs,  or 

Fig.  125. 


13 


posts  which  support  and  insulate  the  parts  of  the  machine.  P 
is  the  glass  plate,  (which  in  some  machines  is  a  hollow  cylin- 
der,) from  which  the  electricity  is  excited,  and  H  is  the  handle 
by  which  the  plate  (or  cylinder)  is  turned.  R  is  a  leather 
cushion,  or  rubber,  held  closely  to  both  sides  of  the  glass  plate 

*  The  amalgam  is  composed  of  mercury,  tin,  and  zinc.  That  recom- 
mended by  Singer,  is  made  by  melting  together  one  ounce  of  tin  and  two 
ounces  of  zinc,  which  are  to  be  mixed,  while  fluid,  with  six  ounces  of 
mercury,  and  agitated  in  an  iron,  or  thick  wooden  box,  until  cold.  It  is 
then  to  be  reduced  to  a  very  fine  powder  in  a  mortar,  and  mixed  with  a 
sufficient  quantity  of  lard  to  form  it  into  a  paste. 

264.  For  what  purpose  is  the  electrical  machine  constructed?  Upon 
what  principle  are  all  electrical  machines  constructed  ?  How  is  the  elec- 
tricity excited?  Of  what  is  the  amalgam  composed?  In  what  form  is 
the  glass  surface  made?  When  is  the  machine  called  a  plate  machine? 
When  is  it  called  a  cylinder  machine?  What  does  Fig.  125  represent? 
Explain  the  figure. 


ELECTRICITY. 


203 


by  a  brass  clasp,  supported  by  the  post  G  L,  wliich  is  called 
the  rubbei'-post.  S  is  a  silk  bag,^*  embraced  by  the  same 
clasp  that  holds  the  leather  cushion  or  rubber ;  and  it  is  con- 
nected by  strings  S  S  S  attached  to  its  three  other  corners,  and 
to  the  legs  LL  and  the  fork  F  of  the  prime  conductor.  C  is 
the  prime  conductor,  terminating  at  one  end  with  a  moveable 
brass  ball,  B,  and  at  the  other  by  the  fork  F,  which  has  one 
prong  on  each  side  of  the  glass  plate.  On  each  prong  of  the 
fork  there  are  several  sharp  points  projecting  towards  the 
plate,  to  collect  the  electricity  as  it  is  generated  by  the  friction 
of  the  plate  against  the  rubber.  Y  is  a  chain,  or  wire,  attach- 
ed to  the  bi^ass  ball  on  the  rubber-post,  and  resting  on  the 
table  or  the  floor,  designed  to  convey  the  fluid  from  the  ground 
to  the  plate.  When  negative  electricity  is  to  be  obtained,  this 
chain  is  removed  from  the  rubber-post,  and  attached  to  the 
prime  conductor,  and  the  electricity  is  to  be  gathered  from  the 
ball  on  the  rubber-post. 

Operation  of  the  Machine. — By  turning  the  handle  H,  the 
glass  plate  is  pressed  by  the  rubber.  The  friction  of  the 
rubber  against  the  glass  plate  (or  cylinder)  produces  a  transfer 
of  the  electric  fluid  from  the  rubber  to  the  plate ;  that  is,  the 
cushion  becomes  negatively  and  the  glass  positively  electrified. 
The  fluid  which  thus  adheres  to  the  glass,  is  carried  round  by 
the  revolution  of  the  cylinder ;  and  its  escape  being  prevented 
by  the  silk  bag,  or  flap,  which  covers  the  plate,  (or  cylinder,) 
until  it  comes  to  the  immediate  vicinity  of  the  metallic  points 
on  the  fork  F,  it  is  attracted  by  the  points,  and  carried  by 
them  to  the  prime  conductor.  Positive  electricity  is  thus 
accumulated  on  the  prime  conductor,  while  the  conductor  on 
the  rubber-post,  being  deprived  of  this  electricity,  is  negatively 
electrified.  The  fluid  may  then  be  collected  by  a  Leyden  jar 
from  the  prime  conductor,  or  conveyed,  by  means  of  a  chain 
attached  to  the  prime  conductor,  to  any  substance  which  is  to 
be  electrified.  If  both  of  the  conductors  be  insulated,  but  a 
small  portion  of  the  electric  fluid  can  be  excited  ;  for  this 
reason,  the  chain  must  in  all  cases  he  attached  to  the  rubber-post^ 
when  positive  electricity  is  required,  and  to  the  prims  conductor, 
when  negative  electricity  is  ivanted, 

*  In  cylindrical  machines  this  silk  bag  is  called  "  the  flap.^^ 

Explain  the  operation  of  the  machine.  To  what  must  the  chain  be  at- 
tached when  positive  electricity  is  required?  To  what  must  it  be  attached 
when  negative  electricity  is  wanted  ? 


204 


NATURAL  PHILOSOPHY. 


EXPERIMENTS  WITH  THE  ELECTRICAL  MACHINE. 

1.  On  the  prime  conductor  of  the  electi  ical  machine  is  placed 
the  electrometer,"^  E.  It  consists  of  a  wooden  ball  mounted 
on  a  metallic  stick,  or  wire,  having  two  pith  balls,  suspended 
by  silk,  hair,  or  linen  threads.  When  the  machine  is  woiked, 
the  pith  balls,  being  both  similarly  electrified,  repel  each 
other ;  and  this  causes  them  to  fly  apart,  as  is  represented  in 
the  figure  ;  and  they  will  continue  elevated  until  the  electricity 
is  drawn  off.  But  if  an  uninsulated  conducting  substance 
touch  the  prime  conductor,  the  pith  balls  will  fall.  The  height 
to  which  the  balls  rise,  and  the  quickness  with  which  they  are 
elevated,  afford  some  test  of  the  power  of  the  machine.  This 
simple  apparatus  may  be  attached  to  any  body,  the  electricity 
of  which  we  wish  to  measure. 

2.  The  balls  of  the  electrometer,  when  elevated,  are  attract- 
ed by  any  resinous  substance,  and  repelled  oy  any  vitreous 
substance  that  has  been  previously  excited  by  friction. 

3.  If  an  electric,  or  a  non-conductor,  be  presented  to  the 
prime  conductor,  when  charged,  it  will  produce  no  effect  on 
the  balls ;  but  if  a  non-electvic,  or  any  conducting  substance 
be  presented  to  the  conductor,  the  balls  of  the  electrometer 
will  fall.  This  shows  that  the  conductor  has  parted  with  its 
electricity,  and  that  the  fluid  has  passed  off  to  the  earth 
through  the  substance,  and  the  hand  of  the  person  present- 
inof  it. 

*  The  word  "  electrometer^^  means  "  a  measurer  of  electricity.'^  It  is 
made  in  a  variety  of  forms,  on  the  principle  that  similar  states  of  elec- 
tricity repel  each  other.  It  sometimes  consists  of  a  single  pith-ball,  at- 
tached to  a  hght  rod,  in  the  manner  of  a  pendulum,  before  a  graduated 
arc  or  circle. 

An  electroscope  is  an  instrument  of  more  delicate  construction,  to  de- 
tect the  presence  of  electricity.  The  most  sensitive  of  this  kind  of  appara- 
tus, is  that  called  Bennett's  Goldleaf  Electroscope,  improved  by  Singer. 
It  consists  of  two  strips  of  goldleaf  suspended  under  a  glass  covering  which 
completely  insulates  them.  Strips  of  tinfoil  are  attached  to  the  sides  of 
the  glass,  opposite  the  goldleaf,  and  when  the  strips  of  goldleaf  diverge, 
they  will  touch  the  tinfoil,  and  be  discharged.  A  pointed  wire  surmounts 
the  instrument,  by  which  the  electricity  of  the  atmosphere  may  be  ob 
served. 

What  is  the  first  experiment  mentioned  witli  the  electrical  machine? 
What  does  the  word  electrometer  meau  ?  Of  what  does  it  sometimes 
consist  ?    Whiit  is  an  electroscope  ?    What  is  the  second  experiment? 


ELECTIIICITY. 


205 


4.  When  tlie  macliine  is  turned,  if  a  person  touch  tlie  piime 
conductor,  the  Ihiid  passes  o(F  through  the  person  to  the  lloor 
without  his  feehng  it.  But  if  lie  present  his  finger,  his 
knuckle,  or  any  part  of  the  body,  near  to  the  conductor,  with- 
out touching  it,  a  spark  will  pass  from  the  conductor  to  the 
knuckle,  which  will  produce  a  sensation  similar  to  the  pricking 
of  a  pin,  or  needle. 

5.  If  a  person  stand  on  a  stool  with  glass  legs,  or  any 
other  non-conductor,  he  will  be  insulated.  If  in  this  situation 
he  touch  the  prime  conductor,  or  a  chain  connected  with  it, 
when  the  machine  is  worked,  sparks  may  be  drawn  from  any 
part  of  the  body  in  the  same  manner  as  from  the  prime  con 
ductor.  While  the  person  remains  insulated,  he  experiences 
no  sensation  from  being  filled  with  electricity ;  or,  if  a  metallic 
point  be  presented  to  any  part  of  his  body,  the  fluid  may  be 
drawn  oif  silently,  without  being  perceived.  But  if  he  touch 
a  blunt  piece  of  metal,  or  any  other  conducting  substance,  or 
if  he  step  from  the  stool  to  the  floor,  he  will  feel  the  electric 
shock ;  and  the  shock  will  vary  in  force  according  to  the 
quantity  of  fluid  with  which  he  is  charged. 

6.  The  Tissue  Figure.  Fig.  126  is  a  figure  with  a  dress  of  fan- 
cy paper  cut  into  narrow  strips.   When  placed  on  the  prime  con- 
ductor, or  being  insulated,  is  con- 
nected Avith   it,   the    strips    being  -^ig-  126. 

all  electrified  will  recede  and  form  a 
sphere  around  the  head.  On  pre- 
senting a  metallic  point  to  the  elec- 
trified strips,  very  singular  combina- 
tions will  take  place.  If  the  elec- 
trometer be  removed  from  the  prime 
conductor,  and  a  tuft  of  feathers,  or 
hair,  fastened  to  a  stick  or  wire,  be 
put  in  its  place,  on  turning  the  ma- 
chine the  feathers  or  hair  will  become 
electrified,  and  the  separate  hairs  will 
rise  and  repel  each  other.  A  toy  is  in  this  way  constructed, 
representing  a  person  under  excessive  fright.  On  touching 
the  head  with  the  hand,  or  any  conducting  substance,  not  in- 
sulated, the  hair  will  fall. 

7.  The  Ley  den  jar  may  be  charged  by  presenting  it  to  the 


What  is  the  tliird  ?  vYiiat  does  this  show  ?  What  is  the  fourth  I  What 
is  the  fifth  ?    What  is  the  sixth  ? 


206 


NATURAL  PHILOSOPHY. 


prime  conductor,  when  the  machine  is  worked.  If  the  ball  of 
the  jar  touch  the  prime  conductor,  it  will  receive  the  fluid 
silently ;  but  if  the  ball  of  the  jar  be  held  at  a  small  dis- 
tance from  the  prime  conductor,  the  sparks  will  be  seen  darting 
from  the  prime  conductor  to  the  jar  with  considerable  noise. 

8.  The  jar  may  in  like  manner  be  filled  with  negative  elec- 
tricity, by  applying  it  to  the  ball  on  the  rubber-post,  and  con- 
necting the  chain  with  the  prime  conductor. 

9.  If  the  Ley  den  jar  be  charged  from  the  prim_e  conductor, 
(that  is,  with  positive  electricity,)  and  presented  to  the  pith 
balls  of  the  electrometer,  they  will  be  repelled ;  but  if  the  jar 
be  charged  from  the  brass  ball  of  the  rubber-post,  (that  is, 
with  negative  electricity,)  they  will  be  attracted. 

10.  If  the  electrometer  be  removed  from  the  prime  con- 
ductor, and  a  pointed  wire  be  substituted  for  it,  a  wire  with 
sharp  points  bent  in  the  form  of  an  S,  balanced  on  it,  will  be 
made  to  revolve  rapidly.    In  a  similar  man- 
ner the  motion  of  the  sun  and  the  earth        Fig.  127 
around  their  common  centre  of  gra\ity,  to- 
gether with  the  motion  of  the  earth  and  the 
moon,  may  be  represented."* 

11.  A  chime  of  small  bells,  on  a  stand, 
Fig.  127,  may  also  be  rung  by  means  of  brass 
balls  suspended  from  the  revolving  wires. 
The  principle  of  this  revolution  is  similar  to 
that  mentioned  in  connexion  with  the  re- 
vohing  jet,  Fig.  85,  which  is  founded  on  the 
law,  that  action  and  reaction  are  equal  and 
in  opposite  directions. 

12.  If  powdered  resin  be  scattered  over  dry  cotton-wool, 
loosely  wrapped  on  one  end  of  the  jointed  discharger,  it  may 
be  inflamed  by  the  discharge  of  the  battery  or  a  Ley  den  jar. 
G-unpowder  may  be  substituted  for  the  resin. 

265.  The  universal  discharger^  is  an  instrument  for 
directing  a  charge  of  electricity  through  any  substance, 
with  certainty  and  precision. 

*  Such  an  apparatus  is  sometimes  called  an  Electrical  Tellurium. 
It  may  rest  on  tiie  prime  conductor,  or  upon  an  insulated  stand. 

What  is  the  seventh?  How  may  the  jar  be  filled  with  negative  elec- 
tricity? What  is  the  eighth?  What  is  the  ninth?  What  is  the  tenth? 
Wliat  is  the  eleventh  ?    What  is  the  twelfth  ? 

265.  What  is  the  universal  discharger?    What  is  its  use? 


El/ECTRICITY. 


207 


1.  It  consists  of  two  slidini,^  rods,  A  B  and  C  D,  terminating 
at  the  extremities,  A  and  D, 

with  brass  balls,  and  at  the  ^ 
other  ends,  which  rest  upon  the 
ivory  table  or  stand  E,  having  a 
fork,  to  which  any  small  sub- 
stance may  be  attached.  The 
whole  is  insulated  by  glass  legs 
or  pillars.  The  rods  shde  through 
collars,  by  which  means  their 
distance  from  one  another  may  be  adjusted. 

2.  In  using  the  universal  discharger,  one  of  the  rods  or 
slides  must  be  connected  by  a  chain,  or,  otherwise,  with  the 
outside,  and  the  other  with  the  inside  coating  of  the  jar  or 
battery.  By  this  means  the  substance  through  which  the 
charge  is  to  be  sent  is  placed  within  the  electric  circuit. 

3.  By  means  of  the  universal  discharger,  any  small  metallic 
substance  may  be  burnt.  The  substance  must  be  placed  in  the 
forks  of  the  slides,  and  the  slides  placed  wdthin  the  electric  cir- 
cuit, in  the  manner  described  in  the  last  paragraph.  In  the 
same  manner,  by  bringing  the  forks  of  the  shdes  into  contact 
w^ith  a  substance  placed  upon  the  ivory  stand  of  the  discharger, 
such  as  an  egg,  a  piece  of  a  potato,  water,  &c.,  it  may  be 
illuminated. 

4.  Ether,  or  alcohol,  may  be  intiamed  by  a  spark  commu- 
nicated from  a  person,  in  the  following  manner.  The  person 
standing  on  the  insulating  stool,  receives  the  electric  fluid  from 
the  prime  conductor,  by  touching  the  conductor  or  any  con- 
ducting substance  in  contact  with  it ;  he  then  inserts  the  knuckles 
of  his  hand  in  a  small  quantity  of  sulphuric  ether,  or  alcohol, 
held  in  a  shallow  metalhc  cup,  by  another  person,  who  is  not 
insulated,  and  the  ether  or  alcohol  immediately  inflames.  In 
this  case  the  fluid  passes  from  the  conductor  to  the  person 
who  is  insulated,  and  he  becomes  charged  with  electricity. 
As  soon  as  he  touches  the  hquid  in  the  cup,  the  electric  tluid, 
passing  from  him  to  the  spirit,  sets  it  on  fire. 


What  figure  represents  it?  Of  what  does  it  consist?  What  is  neces- 
sary in  using  the  universal  discharger?  What  is  effected  by  this  means? 
Wliat  experiments  are  shown  by  means  of  the  universal  discharger? 
How  must  the  substance  be  placed?  How  may  ether  or  alcohol  be  in- 
flamed? 


208 


NATURAL  riilLOSOPHY. 


266.  The  electrical  bells  are  designed  to  show  the 
effects  of  electrical  attraction  and  repulsion. 

1.  Thev  are  thus  to  be  applied  *    The  ball  B  of  the  prime 
conductor,  with  its  rod,  is  to  be  unscrewed,  and  the  rod  on 
which  the  bells  are  suspended  is 
to  be  screwed  in  its  place.    The  Fig.  129- 

middle  bell  is  to  be  connected  by     ©f-^"*'"'   i 

a  L-hain  with  the  table  or  the  1 
floor.  When  the  machine  is  \ 
turned,  the  balls  suspended  be-  j 

tween  the  bellswill  be  alternately  i  n  i  ^  A 
attracted  and  repelled  by  the     A       ^  ^  B 

bells,  and  cause  a  constant  ring-  ^  j 
ing.  If  the  battery  be  charged  j 
and  connected  with  the  prime  • 
conductor,  the  bells  will  continue 

to  ring  until  all  the  fluid  from  the  battery  has  escaped. 

It  may  be  observed,  that  the  fluid  from  the  prime  conductor 
passes  readily  from  the  two  outer  bells,  which  are  suspended  by 
chains ;  they,  therefore,  attract  the  two  balls  towards  them. 
The  balls  becoming  electrified  by  contact  with  the  outer  bells, 
are  repelled  by  them  and  driven  to  the  middle  bell,  to^  which 
they  communicate  their  electricity;  having  parted  ^vith  their 
electricity  they  are  repelled  by  the  middle  bell,  and  again  at- 
tracted by  the  outer  ones,  and  thus  the  constant  ringing^ is 
maintained.  The  fluid  which  is  communicated  to  the  middle 
bell,  is  conducted  to  the  earth  by  the  chain  attached  to  it. 

2.  Spiral  Tube.    The  passage  of  the  electric  fluid  from  one 

Fig.  130. 

conducting  substance  to  another,  is  beautifully  exhibited  by 
means  of  "^a  glass  tube  having  a  brass  ball  at  each  end,  and 

*  In  some  sets  of  instmments,  the  bells  are  insulated  on  a  separate 
stand  ;  but  the  mode  described  above  is  a  convenient  mode  of  connecting 
them  with  the  prime  conductor. 


266.  What  are  the  electrical  bells?  What  figure  represents  tliem  ? 
What  are  they  designed  to  show  ?  How  are  they  to  be  applied  ^  Explain 
the  spiral  tube. 


ELECTRICITY. 


20J 


co:iled  ill  the  iiibiide  with  small  pieces  of  tinfoil,  placed  at  srn;iU 
distanrcs  from  each  other  in  a  spiral  direction,  as  represcriied 
m  Kig.  130. 

In  lhe  same  manner  various  figures,  letters,  and  words  may 
be  represented,  by  ai-ranging  similar  pieces  of  tinfoil  between 
two  pieces  of  flat  glass.  These  experi- 
ments appear  more  brilliant  in  a  darkened 
room. 

3.  Hydrogen  Pistol.  Fig.  131  repre- 
sents a  hydrogen  pistol.*  When  filled  with 
hydrogen  gas,f  if  the  insulated  knob  K  be 
presented  to  the  prime  conductor,  it  will 
immediately  explode. 

4.  Electrical  Sportsman.  Fig.  133  represents  the  electrical 
sportsman.    From  the  larger  ball  of  a  Ley  den  jar  two  birds 


Fig.  131. 


□3= 


=9K 


*  This  apparatus  is  made  in  a  variety  of  forms,  sometimes  in  the  exact 
form  of  a  pistol,  and  sometimes  in  the  form  of  a  piece  of  ordnance.  The 
form  in  the  figure  is  a  simple  and  cheap  contrivance,  and  is  sufficient  to 
explain  the  manner  in  which  the  instrument  is  to  be  used  in  any  of  its 
forms. 

t  A  very  convenient  and  economical  way  of  procuring  hydrogen  gas 
for  this  and  other  experiments,  is  by  means  of  the  hydrogen  gas  genei 
ator,  as  represented  in  Fig.  132.    It  consists  of  a  glass 
vessel,  with  a  brass  cover,  in  the  centre  of  which  is  a 
stop-cock  ;  from  the  inside  of  the  cover,  another  glass 
vessel  is  suspended,  with  its  open  end  downwards.  With- 
in this,  a  piece  of  zinc  is  suspended  by  a  wire.  The 
outer  vessel  contains  a  mixture  of  sulphuric  acid  and 
water,  about  nine  parts  of  water  to  one  of  acid.  When 
the  cover,  to  which  the  inner  glass  is  firmly  fixed,  is 
placed  upon  the  vessel,  the  acid  acting  upon  the  zinc, 
causes  the  metal  to  absorb  the  oxygen  of  the  v/ater,  and 
the  hydrogen,  the  other  constituent  part  of  the  water, 
being  thus  disengaged,  rises  in  the  inner  glass,  from 
which  it  expels  the  water ;  and  when  the  stop-cock  is 
turned  the  hydrogen  gas  may  be  collected  in  the  hydro-  ^ 
gen  pistol,  or  any  other  vessel.    In  the  use  of  hydrogen 
gas  for  explosion,  it  will  be  necessary  to  dilute  the  gas  with  an  equal  por- 
tion of  atmospheric  air. 


Fig.  132, 

I 


Explain  the  hydrogen  pistol. 


210 


NATURAL  PHILOSOPHY. 


Fig.  133 


made  of  pith/*  are  sus- 
pended by  a  linen  thread, 
silk,  or  hair.  When  the 
jar  is  charged  the  birds 
will  rise,  as  represented 
in  the  figure,  on  account 
of  the  repulsion  of  the 
fluid  in  the  jar. 

If  the  jar  be  then  placed 
on  the  tinfoil  of  the  stand, 
and  the  smaller  ball  pla- 
ced within  a  half  inch  of 
the  end  of  the  gun,  a  dis- 
charge will  be  produced,  and  the  birds  will  fall. 


Fig.  134. 


FURTHER  EXPERIMENTS. 

1.  If  images,  made  of  pith,  or  small  pieces  of  paper,  are 
placed  under  the  insulated  stool,  and  a  connexion  be  made 
between  the  prime  conductor  and  the  top  of  the  stool,  the 
images  will  be  alternately  attracted  and  repelled ;  or,  in  other 
w^ords,  they  will  first  rise  to  the  electrified  top  of  the  stool,  and 
thus  becoming  themselves  electrified,  will  be  repelled,  and  fall 
to  the  ground,  the  floor,  or  the  table  ;  where,  parting  with  their 
electricity,  they  will  again  be  attracted  by 
the  stool,  thus  rising  and  falhng  with  con- 
siderable rapidity.  In  order  to  conduct 
this  experiment  successfully,  the  images,  (fee, 
must  be  placed  within  a  short  distance  of 
the  bottom  of  the  stool. 

2.  On  the  same  principle  light  figures  may 
be  made  to  dance  when  placed  between  two 
discs,  the  lower  one  being  placed  upon  a 
sliding  stand  with  a  screw  to  adjust  the  dis- 
tance, and  the  upper  one  being  suspended 
from  the  prime  conductor,  as  in  Fig.  134. 

3.  A  hole  may  be  peiforated  through  a 
quire  of  paper,  by  charging  the  battery,  resting  the  paper  up- 

*  This  substance  is  obtained  in  larue  quantities  from  the  cornstalk,  the 
whole  of  which,  with  the  exception  of  the  outside,  is  composed  of  pith. 


Explain  experiment  one.    Explain  the  second  experiment. 


ELECTRICITY. 


211 


on  the  brass  ball  of  the  battery,  and  making  a  communication, 
by  means  of  the  jointed  discharger,  between  the  ball  oF  one  of 
the  jars  and  the  brass  ball  of  the  box.  Tiie  paper,  in  this  case, 
will  be  between  the  ball  of  the  battery  and  the  end  of  the  dis- 
charger. 

4.  The  thunder-house,  Fig.  135,  is  designed  to  show  the  se- 
curity afforded  by  lightning-rods,  when  lightning  strikes  a 
building.  This  is  done  by  placing 
a  highly  combustible  material  in 
the  inside  of  the  house,  and  pass- 
ing a  charge  of  electricity  through 
it.  On  the  floor  of  the  house  is  a 
surface  of  tinfoil.  The  hydrogen 
pistol  being  filled  with  hydrogen 
gas  from  the  gasometer,  must  be 
placed  on  the  floor  of  the  thunder- 
house,  and  connected  with  the  wire 
on  the  opposite  side.  The  house  being  then  put  together,  a  chain 
must  be  connected  with  the  wire  on  the  side  opposite  to  the 
lightning-rod,  and  the  other  end  placed  in  contact  either  with  a 
single  Ley  den  jar  or  with  the  battery.  When  the  jar,  thus  situ- 
ated, is  charged,  if  a  connexion  be  formed  between  the  jar  and 
the  points  of  the  lightning-rod,  the  fl-uid  will  pass  off  silently,  and 
produce  no  effect.  But  if  a  small  brass  ball  be  placed  on  the 
points  of  the  rod,  and  a  charge  of  electricity  be  sent  to  it,  from 
the  jar  or  the  battery,  the  gas  in  the  pistol  will  explode,  and 
throw  the  parts  of  the  house  asunder  with  a  loud  noise.* 

*  The  success  of  this  experiment  depends  upon  the  proper  connexion 
of  the  jar  with  the  hghtning-rod,  and  the  electrical  pistol.  On  the  side  of 
the  house  opposite  to  the  lightning-rod  there  is  a  wire,  passing  through  the 
side,  and  terminating  on  the  outside  in  a  hook.  When  the  house  is  put 
together,  this  wire,  in  the  inside,  must  touch  the  tinfoil  on  the  floor  of  the 
house.  The  hydrogen  pistol  must  stand  on  the  tinfoil,  and  its  insulated 
knob  or  wire,  projecting  from  its  side,  must  be  connected  with  the  low^er 
end  of  the  hghtning-rod,  extending  into  the  inside  of  the  house.  A  com- 
munication must  then  be  made  betw^een  the  hook  on  the  outside  of  the 
house,  and  the  outside  of  the  jar  or  battery.  This  is  conveniently  done  by 
attaching  one  end  of  a  chain  to  the  hook,  and  holding  the  other  end  in  the 
hand  against  the  side  of  a  charged  jar.  By  presenting  the  knob  of  tho 
jar  to  the  points  of  the  lightning-rod  no  effect  is  produced,  but  if  a  brass 
ball  be  placed  on  the  points  at  P,  and  the  knob  of  the  jar  be  presented  to 


□ 
D 


□ 


DUlDD 


Explain  the  third  experiment.    The  fourth 


212 


NATURAL  PHILOSOPHY. 


5.  If  the  ball  of  the  prhiie  conductor  be  removed  and  a 
pomted  wne  be  put  in  its  place,  the  cuiTent  of  electricity 
flowing  from  the  point,  vrhen  the  machine  is  turned,  may  be 
perceived  by  placing  a  lighted  lamp  before  it ;  the  flame  will  be 
blown  from  the  point;  and  this  will  be  the  case  in  what  part 
soever  of  the  machine  the  point  is  placed,  whether  on  the  prime 
conductor  or  the  rubber ;  or  if  the  point  be  held  in  the  hand 
and  the  flame  placed  between  it  and  the  machine,  thus  show- 
ing that  in  all  cases  the  fluid  is  blown  from  the  point.  Delicate 
apparatus  may  be  put  in  motion  by  the  electric  fluid  when 
issuing  from  a  point.  In  this  way  electrical  orreries,  mills, 
&c.,  are  constructed. 

6.  Goldleaf  may  be  forced  into  the  pores  of  glass  by 
placing  it  between  two  slips  of  window-glass,  pressing  the 
shps  of  glass  firmly  together,  and  sending  a  shock  from  a  bat- 
tery through  them. 

7.  If  goldleaf  be  placed  between  two  cards,  and  a  strong 
charge  be  passed  through  them,  it  will  be  completely  fused. 

8.  When  electricity  enters  at  a  point,  it  appears  in  the  form 
of  a  star ;  but  when  it  goes  out  from  a  point,  it  puts  on  the 
appearance  of  a  brush. 

GENERAL  REMARKS. 

1.  Lightning  is  the  rapid  motion  of  vast  quantities  of  elec- 
tric matter.  Thunder  is  the  noise  produced  by  the  rapid  mo 
tion  of  lightning  through  the  air. 

2.  The  aurora  borealis  (or  northern  hghts)  is  supposed  to  be 
caused  by  the  electric  fluid  passing  through  highly-rarefied 
air ;  and  most  of  the  great  convulsions  of  nature,  such  as 
earthquakes,  whirlwinds,  hurricanes,  water-spouts,  &c.,  are 
generally  accompanied  by  electricity,  and  often  depend  upon  it. 

3.  The  electricity  which  a  body  manifests  by  being  brought 
near  to  an  excited  body,  without  receiving  a  spark  from  it,  is 

the  ball,  the  explosion  will  take  place.  If  the  charged  jar  be  very  sud- 
denly presented  to  the  points,  the  explosion  may  take  place  ;  and  the  jar 
may  be  silently  discharged  if  it  be  brought  very  slowly  to  the  ball.  The 
thunder-house  is  sometimes  put  together  with  magnets. 


Explain  the  fifth  experiment.    The  sixth.    The  seventh. 
What  is  lightning  ?    What  is  thunder  ?    How  is  the  aurora  borealis 
supposed  to  be  caused? 


ELECTIUCITV. 


213 


«aid  to  be  acquired  by  induction.  Whoii  an  insulated  but  un- 
electrified  conductor  is  brouglit  near  an  insulated  charged  con- 
ductor, tlie  end  near  to  tlie  exciled  conductor  assumes  a  state 
of  opposite  electricity,  while  the  farther  end  assumes  the  same 
kind  of  electricity, — that  is,  if  the  conductor  be  electrified 
positively,  the  unelectrified  conductor  will  be  negative  at  the 
nearer  end  and  positive  at  the  further  end,  while  the  middle 
point  evinces  neither  positive  nor  negative  electricity. 

4.  The  experiments  which  have  now  been  described  exem- 
plify all  the  elementary  principles  of  the  science  of  electricity. 
These  experiments  may  be  varied,  multiplied,  and  extended  in 
innumerable  forms,  by  an  ingenious  practical  electrician.  Among 
other  things  with  which  the  subject  may  be  made  interesting, 
may  be  mentioned  the  following  facts,  &c. 

5.  A  number  of  feathers,  suspended  by  strings  from  an  in- 
sulated conducting  substance,  will  rise  and  present  the  ap- 
pearance of  a  flight  of  birds.  As  soon  as  the  substance  is  dis- 
charged the  feathers  will  fall.  The  experiment  may  be  varied 
by  placing  the  sportsman  on  the  prime  conductor,  without  the 
use  of  the  Leyden  jar,  to  which  the  birds  are  attached. 

6.  Instead  of  the  Leyden  jar  a  plate  of  common  glass  (a 
pane  of  window-glass,  for  instance)  may  be  coated  on  both 
sides  with  tinfoil,  leaving  the  edges  bare.  A  bent  wire 
balanced  on  the  edge  of  the  glass,  to  the  ends  of  which  balls 
may  be  attached,  with  an  image  at  each  end,  may  be  made  to 
represent  two  persons  tilting,  on  the  same  principle  by  which 
the  electrical  bells  are  made  to  ring. 

V.  A  beautiful  little  sawmill  was  lately  exhibited  at  a  lecture 
at  the  Odeon,  in  Boston,  by  Mr.  Quimby,  its  ingenious  con- 
triver. The  moving  power  was  a  wheel,  with  balls  at  the  ends 
of  the  spokes,  situated  within  the  attractive  influence  of  two 
larger  balls,  difl'erently  electrified.  As  the  balls  on  the  spokes 
were  attracted  by  one  of  the  lai-ger  balls,  they  changed  their 
electrical  state  and  were  attracted  by  the  other,  which,  in  its 
return,  repelled  them,  and  thus  the  motion  being  given  to  the 
wheel  was  communicated  by  cranks  at  the  end  of  the  axle  to 
the  saws  above. 


How  is  the  electricity  which  a  body  manifests  by  being  brought  near 
to  an  excited  body,  without  receiving  a  spark  from  it,  said  to  be  ac- 
quired? When  an  insulated,  but  imelectrified  conductor,  is  brought  near 
an  insulated  charged  conductor,  what  is  said  with  regard  to  the  end  near 
the  excited  conductor?    What  example  is  given  to  illustrate  this? 


214 


NATURAL  PHILOSOPHY. 


8.  ^Vhen  the  hand  is  presented  to  the  prime  conductor,  a 
spark  is  communicated,  attended  with  a  shghtly  painful  sensa- 
tion. But  if  a  pin  or  a  needle  be  held  in  the  hand  with  the 
point  towards  the  conductor,  neither  spark  nor  pain  will  be 
perceived,  owing  to  the  attracting  (or  perhaps,  more  properly 
speaking,  the  receiving)  'power  of  the  point. 

9.  That  square  rods  are  better  than  round  ones  to  conduct 
electricity  silently  to  the  ground,  and  thus  to  protect  buildings, 
may  be  proved  by  causing  each  kind  of  rod  to  approach  the 
prime  conductor  when  charged.  It  will  thus  be  perceived, 
that  while  httle  effect  is  produced  on  the  pith-balls  of  the 
electrometer  by  the  near  approach  of  the  round  rod,  on  the 
approach  of  the  square  one  the  balls  will  immediately  fall. 
The  round  rod  also,  will  produce  an  explosion  and  a  spark, 
from  the  ball  of  the  prime  conductor,  while  the  square  one  will 
draw  off  the  fluid  silently. 

10.  The  effects  of  pointed  conductors  upon  clouds  charged 
with  electricity  may  be  familiarly  exemphfied  by  suspending  a 
small  fleece  of  cotton-wool  from  the  prime  conductor,  and 
oth^r  smaller  fleeces  from  the  upper  one,  by  small  filaments. 
On  presenting  a  point  to  them  they  will  be  repelled  and  all 
drawn  together ;  but  if  a  blunt  conductor  approach  them  they 
will  be  attracted. 

11.  From  a  great  variety  of  facts,  it  has  been  ascertained, 
that  lightning-rods  afford  but  little  security  to  any  part  of  a 
building  beyond  twenty  feet  from  them ;  and  that  when  a  rod 
is  'painted  it  loses  its  conducting  power.  The  lightning-rods  of 
the  most  approved  construction,  and  in  strictest  accordance 
with  philosophical  principles,  are  composed  of  small  square 
rods,  (similar  to  nail-rods.)  They  run  over  the  building,  and 
down  each  of  the  corners,  presenting  many  elevated  points  in 
their  course.  At  each  of  the  corners,  and  on  the  chimneys,  the 
rods  should  be  elevated  several  feet  above  the  building.  Rods 
of  this  description  have  been  placed  on  all  the  public  school- 
houses  and  other  pubhc  buildings  of  Boston,  by  order  of  the 
city  authorities.  They  were  constructed  by  Dr.  King,  who 
has  introduced  an  improvement,  by  twisting  the  square  rods. 


Why  are  square  rods  better  than  round  ones  to  conduct  electricity  si- 
lently to  the  ground,  and  thus  protect  buildings  from  lightning?  How 
far  beyond  the  rod  do  lightning-rods  afford  protection?  In  what  way  are 
thv.'  jiio^a  approved  lightning-rods  constructed  \ 


ELECTRICITY. 


215 


and  thus  muhiplying  (lie  sliarp  surfaces  presented  to  collect 
the  (luid. 

12.  The  removal  of  silk  and  woollen  garments,  worn  during 
the  day  in  cold  weather,  is  often  accompanied  by  a  slight  noise, 
resembling  that  of  sparks  issuing  from  a  fire.  A  similar 
effect  is  produced  on  passing  the  hand  softly  over  the  back  of 
a  cat.    These  effects  are  produced  by  electricity. 

13.  It  may  here  be  remarked,  that  the  terms  positive  and 
negative,  are  merely  relative  terms,  as  applied  to  the  subject 
of  electricity.  Thus,  a  body  which  is  possessed  of  its  natural 
share  of  electricity,  is  positive  in  respect  to  one  that  has  less, 
and  negative  in  respect  to  one  that  has  more  than  its  natural 
share  of  the  fluid.  So,  also,  one  that  has  more  than  its  natural 
share  is  positive  with  regard  to  one  that  has  only  its  natural 
share,  or  less  than  its  natural  share, — and  negative  in  respect 
to  one  having  a  larger  share  than  itself. 

14.  The  experiments  with  the  spiral  tube  connected  with 
Fig.  130  may  be  beautifully  varied  by  having  a  collection  of 
such  tubes  placed  on  a  stand ;  and  a  jar  coated  with  small 
strips  resembling  a  brick  wall,  presents,  when  it  is  charged,  a 
beautiful  appearance  in  the  dark, 

15.  The  electric  fluid  occupies  no  perceptible  space  of  time 
in  its  passage  through  its  circuit.  It  always  seems  to  prefer 
the  shortest  passage,  when  the  conductors  are  equally  good. 
Thus,  if  two,  ten,  a  hundred,  or  a  thousand  or  moi  e  persons, 
join  hands  and  be  made  part  of  the  circuit  of  the  fluid  in  pass- 
ing from  the  inside  to  the  outside  of  a  Ley  den  jar,  they  will 
all  feel  the  shock  at  the  same  moment  of  time.  But,  in  its 
passage,  the  fluid  always  prefers  the  best  conductors.  Thus, 
if  two  clouds,  differently  electrified,  approach  one  another,  the 
fluid,  in  its  passage  from  one  cloud  to  the  other,  will  some- 
times take  the  earth  in  its  course,  because  the  air  is  a  bad 
conductor. 

16.  In  thunder-storms,  the  electric  fluid  sometimes  passes 
from  the  clouds  to  the  earth,  and  sometimes  from  the  earth  to 
the  clouds ;  and  sometimes,  as  has  just  been  stated,  from  one 
cloud  to  the  earth,  and  from  the  earth  to  another  cloud. 


What  is  remarked  with  regard  to  the  terms  negative  and  positive  ?  How 
can  this  be  illustrated?  What  is  said  with  regard  to  the  time  the  electric 
fluid  occupies  in  its  passage  through  its  circuit?  What  example  is  given 
to  show  that  the  fluid  prefers  the  best  conductors?  In  what  different  ways 
does  tiie  electric  fluid  sometimes  pass  in  thunder-storms? 


216  NATURAL  PHILOSOPHY. 

17  It  is  not  safe,  during  a  thunder- stoim,  to  take  shelter 
under  a  tree,  because  the""  tree  attracts  the  fluid,  and  the 
human  body  being  a  better  conductor  than  the  tree,  the  fluid 
will  leave  the  tree  and  pass  into  the  body. 

18.  It  is  also  unsafe  to  hold  in  the  hand  edge-tools,  or  any 
sharp  point  which  will  attract  the  fluid. 

19  The  safest  position  that  can  be  chosen  during  a  thunder- 
storm, is  a  recumbent  posture  on  a  feather  bed ;  and  ni  all 
situations  a  recumbent  is  safer  than  an  erect  position.  iNo 
dano-er  is  to  be  apprehended  from  lightning  when  the  interval 
between  the  flash  and  the  noise  of  the  explosion  is  as  much 
as  three  or  four  seconds.  This  space  of  time  may  be  con- 
veniently measured  by  the  beatings  of  the  pulse,  if  no  time- 
piece be  at  hand.  17     ir  +^ 

20  LiglUning-rods  were  first  proposed  by  Dr.  J^ranklin  to 
whom  is  also  ;.scribed  the  honor  of  the  discovery  that  thunder 
and  lightning  are  the  effects  of  electncity.  He  raised  a  kite, 
constrScted  of  a  silk  handkerchief,  adjusted  to  two  light  strips 
of  cedar,  with  a  pointed  wire  fixed  to  it ;  and  fastening  the 
end  of  the  twine  to  a  key,  and  the  key,  by  means  of  a  piece  ot 
silk  lace,  to  a  post,  (the'silk  lace  serving  to  insulate  the  whole 
apparatus.)  on  the  approach  of  a  thunder-cloud,  he  was  able 
to  collect  sparks  from  the  key,  to  charge  Leyden  jars,  and  to 
set  fire  to  spirits.  This  experiment  estaWi.shed  the  identity  ot 
htrhtning  and  electricity.  The  experiment  was  a  dangerous  one, 
as  was  proved  in  the  case  of  Professor  Richman,  of  St.  Peters- 
burgh,  who  fell  a  sacrifice  to  his  zeal  for  electrical  science,  by 
a  stroke  of  lightning  from  his  apparatus. 

21  Amonf)-  the  most  remarkable  facts,  connected  willi  the 
science  of  Electricity,  may  be  mentioned  the  power  possessed 
by  certain  species  of  fishes,  of  giving  shocks,  similur  to  those 
produced  by  the  Leyden  jar.  There  are  three  animals  possess- 
ed of  this  power,  namely,  the  Torpedo,  the  Gymnotu.s  Elec- 
tricus,  (or  Surinam  Eel,)  and  the  Silurus  Electricus.  x>ut  aJ- 
thouo-h  it  has  been  ascertained  that  the  Torpedo  is  capaole  ot 
o-ivin'^  shocks  to  the  animal  system,  simil  ir  to  those  ot  the 
Leyden  jar,  yet  he  has  never  been  made  to  afford  a  spark,  nor 

Why  is  it  unsafe,  during  a  thunder-storm,  to  talie  shelter  under  a  tree, 
or  to  hold  in  the  hand  any  edge-tools?  What  position  is  the  safest  m  a 
thunder-storm?  Wlien  is  there  no  danger  to  be  apprehended  from  the 
lightning?  By  whom  were  lightning-rods  first  proposed ?  Who  first  dis- 
covered^that  thunder  and  lightning  are  the  effects  of  electricity  ? 


ELECTRICITY. 


217 


to  produce  the  least  effect  upon  tlie  most  delicate  electrometer. 
The  Gymnotus  gives  a  small  but  perceptible  spark.  'J'he  elec- 
trical powers  of  the  Silurus  are  inferior  to  those  of  the  Torpedo 
or  the  Gymnotus,  but  still  sufficient  to  give  a  distinct  shock  to 
the  human  system.  This  power  seems  to  have  been  bestow^ed 
upon  these  animals  to  enable  them  to  secure  their  piey ;  and 
to  resist  the  attacks  of  their  enemies.  Small  fishes,  when  put 
into  the  water  where  the  Gymnotus  is  kept,  are  generally  killed 
or  stunned  by  the  shock  and  swallowed  by  the  animal,  when 
he  is  hungry.  The  Gymnotus  seems  to  be  possessed  of  a  new 
kind  of  sense,  by  which  he  perceives  whether  the  bodies  pie- 
sented  to  him  be  conductors  or  not."^ 

22.  It  will  be  recollected  that  the  phenomena  which  have 
now  been  described,  with  the  exception  of  what  has  just  been 
stated  as  belonging  to  animal  electricity,  belong  to  the  subject 
oi  frictional  electricity.  But  there  are  other  forms  in  which 
this  subtle  agent  presents  itself,  which  are  yet  to  be  described, 
which  show  that  its  operations  are  not  confined  to  beauliful 
experiments  such  as  have  already  been  presented,  nor  to  the 
terrific  and  tremendous  effects  that  we  witness  in  the  storm 
and  the  thunder-gust.  Its  powerful  agency  works  unseen  on 
the  intimate  relations  of  the  parts  and  properties  of  bodies  of 
every  description,  effecting  changes  in  their  constitution  and 
character,  so  wonderfully  minute,  thorough,  and  universal,  that 
it  may  almost  be  considered  as  the  chief  agent  of  nature, 
the  prime  minister  of  omnipotence,  the  vicegerent  of  creative 
power. 

*  The  consideration  of  the  electricity  developed  by  the  organs  of  these 
animals  of  the  aquatic  order,  belongs  to  that  department  called  Animal 
Electricity. 


10 


218 


NATURAL  PHILOSOPHY. 


CHAPTER  XII, 

GALVANISM,  OR  VOLTAIC  ELECTRICITY. 

267.  Galvanism,  a  branch  of  Electricity,  derives  its 
name  from  Galvani,*  who  first  discovered  the  principles 
which  form  its  basis. 

1.  Electricity  is  produced  by  the  mechanical  action  of  bodies 
on  one  another  ;  but  Galvanism,  or  Galvanic  Electricity,  is  pro- 
duced by  their  chemical  action. 

2.  The  motion  of  the  electric  fluid  excited  by  galvanic 
power,  differs  from  that  explained  rmder  the  head  of  fiictional 
elect  deity,  in  its  duration ;  for  while  the  latter  exhibits  itself  in 
sudden  and  intermitted  shocks  and  explosions,  the  former  con- 
tinues in  constant  and  uninterrupted  action. 

3.  The  nerves  and  muscles  of  animals  are  most  easily 

*  Dr.  Aioysiiis  Galvani  was  a  Professor  of  Anatomy  in  Bologna,  and 
made  his  discoveries  about  the  year  1790.  His  wife,  being  consamp- 
tive,  was  advised  to  take,  as  a  nutritive  article  of  diet,  some  soup  m.ade  of 
the  flesh  of  frogs.  Several  of  these  animals,  recently  skinned  for  that  pur- 
pose, were  lying  on  a  table  in  his  laboratory,  near  an  electrical  machine, 
with  which  a  pupil  of  the  professor  was  amusing  himself  in  trying  experi- 
ments. While  the  machine  was  in  action,  he  chanced  to  touch  the  bare 
nerve  of  the  leg  of  one  of  the  frogs  with  the  blade  of  a  knife  that  he  held 
in  his  hand,  when  suddenly  the  whole  limb  was  thrown  into  violent  con- 
vulsions. Galvani  being  informed  of  the  fact,  repeated  the  experiment,  and 
examined  minutely  all  the  circumstances  connected  with  it.  In  this  way 
he  was  led  to  the  discovery  of  the  principles  which  form  the  basis  of  this 
science.  The  science  was  subsequently  extended  by  the  discoveries  of 
Professor  Volta,  of  Pavia,  who  first  constructed  the  Galvanic  or  V oltaic 
pile,  in  the  beginning  of  the  present  century. 

To  produce  electricity  mechanically,  (as  has  been  stated  under  the  head 
of  frictional  electricity,)  it  is  necessary  to  excite  an  electric  or  non-con- 
ducting substance  by  friction.  But  galvanic  action  is  produced  by  the 
contact  of  difierent  conducting  substances  with  each  other. 

267.  What  is  Galvanism?  By  whom  and  when  was  galvanism  dis- 
covered? What  led  to  the  discovery?  How  is  electricity  generally  pro- 
duced? How  is  galvanic  electricity  produced?  flow  does  the  motion  of 
the  galvanic  fluid,  excited  by  galvanic  power,  differ  from  that  explained 
in  the  science  of  Electricity?  What  bodies  are  most  easily  affected  by 
the  galvanic  fluid  ? 


GALVANiaiVT. 


219 


afToctod  by  the  galvanic  fluid  ;  and  tlic  voltaic  or  galvanic 
batteiy  possesses  the  most  surprising  powers  of  chemical 
decomposition. 

268.  The  galvanic  fluid  or  influence  is  excited  by  tne 
contact  of  pieces  of  different  metal,  and  sometimes  by 
difl^erent  pieces  of  the  same  metal. 

1.  If  a  living  frog,  or  a  fish,  having  a  slip  of  tinfoil  on  its 
back,  be  placed  upon  a  piece  of  zinc,  spasms  of  the  muscles 
Will  be  excited  whenever  a  communication  is  made  between  the 
?:iiic  and  the  tinfoil. 

2.  If  a  person  place  a  piece  of  one  metal,  as  a  half-dollar, 
jibove  his  tongue,  and  a  piece  of  some  other  metal,  (as  zinc,) 
below  the  tongue,  he  will  perceive  a  peculiar  taste ;  and,  in 
the  dark,  will  see  a  flash  of  light,  wlienever  the  outer  edges  of 
the  metals  are  in  contact. 

3.  A  faint  flash  may  be  made  to  appear  before  the  eyes  by 
putting  a  shp  of  tinfoil  upon  the  bulb  of  one  of  the  eyes,  a 
piece  of  silver  in  the  mouth,  and  making  a  communication  be- 
tAveen  them.  In  these  experiments,  no  eflfect  is  produced  so 
long  as  the  metals  are  kept  apart ;  but  on  bringing  them  into 
contact,  the  effects  above  described  .are  produced. 

269.  The  conductors  of  the  galvanic  fluid,  like  those 
of  frictional  electricity,  are  divided  into  the  perfect  and 
the  imperfect.  Metallic  substances,  plumbago  and  char- 
coal, the  mineral  acids  and  saline  solutions,  are  perfect 
conductors.  Water,  oxydated  fluids,  as  the  acids,  and 
all  the  substances  that  contain  these  fluids,  alcohol, 
ether,  sulphur,  oils,  resins,  and  metallic  oxydes,  are 
imperfect  conductors. 

270.  To  produce  any  galvanic  action  it  is  necessary 
to  form  v^hat  is  called  a  galvanic  circle  ;  that  is,  a  cer- 
tain order  or  succession  of  substances  capable  of  exciting 
electricity. 


268.  How  is  the  galvanic  fluid  or  influence  excited  ?  What  illustrations 
of  this  are  given  ? 

269.  Into  what  are  the  conductors  of  the  galvanic  fluid  divided?  What 
Kuhstances  are  perfect  conductors?  What  substances  are  imperfect  con 
ductors  ? 

270.  What  is  necessary  in  order  to  produce  galvanic  action  ? 


220 


NATURAL  PHILOSOPHY. 


271.  The  simplest  galvanic  circle  is  composed  of 
three  conductors,  one  of  which  must  be  solid,  and  one 
fluid  ;  the  third  may  be  either  solid  or  fluid. 

1.  The  process  usually  adopted  for  obtaiuing  galvanic  elec- 
tricity is  to  place  between  two  plates,  of  different  kinds  of 
uetal,  a  fluid  capable  of  exerting  some  chemical  action  on  one 
cf  the  plates,  while  it  has  no  action,  or  a  dii^erent  action,  on 
the  other.  A  communication  is  then  formed  between  the  two 
plates. 

2.  Fio-.  136  represents  a  simple 
galvanic^  circle.  It  consists  of  a 
vessel  containing  a  portion  of  diluted 
sulphuric  acid,'^  with  a  plate  of  zinc 
Z  and  of  copper  C  immersed  in  it. 
The  plates  are  separated  at  the  bot- 
tom, and  the  circle  is  completed  by 
connecting  the  two  plates  on  the 
outside  of  the  vessel  by  means  of 
wires.  The  same  effect  will  be  pro- 
duced, if,  instead  of  using  the  wires 
the  metallic  plates  come  ^.nto  direct 
contact. 

3.  In  the  above  aiTangement, 
there  are  three  elements  or  essen- 
tial parts  ;t  namely,  the  zinc,  the 
copper,  and  the  acid.  The  acid,  act- 
ing chemically^  upon  the  zinc,  pro- 
dtices  an  alteration  in  the  electrical 
state  of  the  metal.    The  zinc  com- 

*  The  acid  employed  iu  the  galvanic  circuit  must  always  be  one  that 
has  a  strong  affinity*  for  one  of  the  metals  in  the  circuit.  When  zinc  is 
employed,  sulphuric  acid  may  form  one  of  the  three  elements,  because 
that  acid  has  a  strong  affinity  for  zinc. 

t  It  is  essential  in'^all  cases  to  have  tliree  elements  to  produce  galvanic 
action.  In  the  experiments  which  have  already  been  mentioned,  in  the 
case  of  the  fro^s,  the  fish,  the  mouth,  and  the  eye,  the  moisture  of  the 
animal,  or  of  the  mouth,  supplies  the  place  of  the  acid,  so  that  the  three 
constituent  parts  of  the  circle  are  completed. 

X  A  certain  quantity  of  electricity  is  always  developed  whenever 

271.  Of  what  is  the  simplest  galvanic  circle  composed  ?  What  process 
18  usually  adopted  for  obtaining  galvanic  electricity  ?  Illustrate  this  by 
Fig.  136. 


Fig.  136. 


GALVANISM. 


221 


munii'atin:;-  its  natural  share  of  the  electrical  fluid  to  the  acid,  be- 
comes neyativdy'^  electrified.  The  coppei-,  attracting  the  same 
fluid  from  tlie  acid,  becomes  positivebj  electrified.  Any  con- 
duclinij^  substance,  therefore,  placed  witliin  tlic  hne  of  commu- 
nication between  the  positive  and  negative  points,  will  receive  the 
charge  thus  to  be  obtained.  The  arrows  ia  Fig.  136  show  the 
direction  of  the  current  of  positive  electricity,  namely,  from 
the  zinc  to  the  fluid, — from  the  fluid  to  the  copper, — from  the 
copper  back  through  the  wires  to  the  zinc,  passing  from  zinc 
to  copper  in  the  acid,  and  from  copper  to  zinc  out  of  the  acid. 
The  substance  submitted  to  the  action  of  the  electric  current 
must  be  placed  in  the  line  of  communication  between  the  cop- 
per and  the  zinc.  The  wire  connected  with  the  copper  is 
called  the  positive  pole,  and  that  connected  with  the  zinc  the 
negative  pole,  and  in  all  cases  the  substance  submitted  to  galvanic 
action  must  be  placed  betvjeen  the  positive  and.  negative  poles. 

4.  The  electrical  eflects  of  a  simple  galvanic  circle,  such  as 
has  now  been  described,  are,  in  general,  too  feeble  to  be  per- 
ceived, except  by  very  delicate  tests.    The  muscles  of  animals, 

chemical  action  takes  place  between  a  fluid  and  a  solid  body.  This  is  a 
general  law  of  chemical  action  ;  and,  indeed,  it  has  been  ascertained,  that 
there  is  so  intimate  a  connexion  between  electrical  and  chemical  changes, 
that  the  chemical  action  can  proceed  only  to  a  certain  extent,  unless  the 
electrical  equilibrium,  which  has  been  disturbed,  be  again  restored.  Hence, 
we  find  that  in  the  simple,  as  well  as  in  the  compound  galvanic  circle,  the 
oxydation  of  the  zinc  proceeds  with  activity  whenever  the  galvanic  circle 
is  completed  ;  and  that  it  ceases,  or,  at  least,  takes  place  very  slowly, 
whenever  the  circuit  is  interrupted. 

*  It  appears  at  first  view  to  be  a  singular  fact,  that  in  a  simple  galvanic 
circle,  composed  of  zinc,  acid,  and  copper,  the  zinc  end  will  always  be 
negative  and  the  copper  end  positive  ;  while  in  all  compound  galvanic  cir- 
cles, composed  of  the  same  elements,  the  zinc  will  be  positive,  and  the 
copper  negative.  This  apparent  difference  arises  from  the  compound  circle 
being  usually  terminated  by  two  superfluous  plates. 


What  effect  will  be  produced  if,  instead  of  allowing  the  metallic  plates 
to  come  into  direct  contact,  the  communication  between  them  be  effected 
by  wires?  How  many  parts  are  there  in  the  above  arrangement?  What 
are  they?  What  effect  does  the  acid  produce?  What  is  the  electrical 
stcte  of  the  zinc?  Of  the  copper?  What  are  the  arrows  in  Fig.  136 
designed  to  shov/ ?  Where  must  the  substance,  to  be  submitted  to  the 
action  of  the  fluid,  be  placed  ?  What  is  said  of  the  electrical  effects  of  a 
simple  galvanic  circle  ? 


222  NATURAL  PHILOSOPHY. 

especially  those  of  cold-blooded  animals,  such  as  frogs.  &c., 
the  tongue,  the  eye,  and  other  sensitive  parts  of  the  body,  be- 
ing very  easily  affected,  afford  examples  of  the  operation  of 
simple  galvanic  circles.  In  these,  aUhough  the  quantity  of 
electricity  set  in  motion  is  exceedingly  small,  it  is  yet  sufficient 
to  produce  very  considerable  effects ;  but  it  produces  little  or 
no  effect  on  the  most  delicate  electromeier. 

272.  The  galvanic  effects  of  a  simple  circle  may  be 
increased  to  any  degree,  by  a  repetition  of  the  samt 
simple  combination. 

Such  repetitions  constitute  compound  galvanic  circles,  and 
are  called  galvanic  piles  or  galvanic  batteries,  according  to  the 
mode  in  which  they  are  constructed. 

273.  The  voltaic  pile  consists  of  alternate  plates  of 
two  different  kinds  of  metal,  separated  by  woollen  cloth, 
card,  or  some  similar  substance. 

1.  Fig.  137  represents  a  voltaic  pile.    A  voltaic  pile  may 
be  constructed  in  the  following  manner :  take  a  number  of 
plates  of  silver,  and  the  same  number  of 
zinc,  and  also  of  woollen  cloch,  the  cloth  Fig.  137. 


having  been  soaked  in  a  solution  of  sal 
ammoniac  in  water  ;  with  these  a  pile  is  to 
be  formed  in  the  following  order  :  namely, 
a  piece  of  silver,  a  piece  of  zinc,  a  piece  of 
cloth,  and  thus  repeated.  These  are  to  be 
supported  by  three  glass  rods,  placed  per- 


pendicularly with  pieces  of  wood  at  the 
top  and  bottom,  and  the  pile  will  then  be  complete ;  and  will 
afford  a  constant  current  of  electric  fluid  through  any  conduct- 
ing substance.  Thus,  if  one  hand  be  applied  to  the  lower 
plate,  and  the  other  to  the  upper  one,  a  shock  will  be  felt, 
which  will  be  repeated  as  often  as  the  contact  is  renewed. 
Instead  of  silver,  copper  plates,  or  pLUes  of  other  met  .1, 


What  examples  are  given  iJliistrat'iiig  the  operation  of  simple  galvanic 
circles? 

272.  How  may  the  galvanic  effects  of  the  simple  circle  be  increased  ? 
What  are  compound  galvanic  circles  ? 

273.  Of  what  does  the  voltaic  pile  consist?  What  does  Fig.  137  rep- 
resent? How  may  a  voltaic  pile  be  constructed?  Can  any  other  metui 
be  used?    What  are  the  arrows  in  the  figure  designed  to  show  ? 


C  \  I,  VAN  ISM. 


may  bi'  used  in  ihc  a!>.>\  i'  ai-i  aii^i'inciil .  TIk;  ari-ows  in  (Jh; 
liguri'.  show  I  he  roursc  of  the  oiii  iciil  of  clcclricily  in  ar- 
rangi'nu'iit  of  silver,  zinc,  A'c. 

2.  \'ollaii'  j)ili's  have  hern  const luclcd  of  la^eis  of  o-olJ  jind 
sil\  (M-  paper.  The  ellccL  of  such  piles  remains  undistui  bed  for 
years.  \\  illi  tlie  assistance  of  two  such  piles,  ;i  kind  of  ^xr- 
pctual  ))U)iio)i,  or  self-moving  clock,  has  been  invented  by  an 
Italian  piiilosophe: .  The  motion  is  produced  by  the  attriiciion 
and  repulsion  of  the  piles  exerted  on  a  pith  ball,  on  the  prm- 
cij)le  of  the  electrical  bells.  The  top  of  one  of  the^  piles  was 
positive,  and  the  bottom  negative.  The  other  pile  was  in  an 
o[)posite  state ;  namely,  the  top  negative,  and  the  bottom 
positive. 

271.  The  voltaic  or  galvanic  battery  is  a  combination 
of  metallic  plates,  immersed  by  pairs  in  a  fluid  which 
exerts  a  chemical  action  on  one  of  each  pair  of  the 
plates,  and  no  action,  or,  at  least,  a  different  action  on 
the  other.* 

1.  Fig.  138  represents  a  voltaic  battery.    It  consists  of  a 
trough  made  of  baked  wood,  wedgewood-ware,  or  some  other 
non-conducting  substance.    It  is  divided  into  grooves  or  par- 
titions, tor  the  reception  of  the 
acid,  or  a  sahne  solution,  and  the 


plates  of  zinc  or  copper  (or  other 
metals)  are  immersed  by  pairs  in 
the  grooves.  These  pairs  of  plates 


are  iiiiited  by  a  slip  of  metal  pass- 
ing 1.  oni  tiie  one  and  soldered  to 
the  other  ;  each  pair  being  placed 
so  as  to  enclose  a  partition  be- 
tween them,  and  eacii  cell  or 
o';o3ve  in  t-se  trouo^h  containinof 

*  The  electricity  excited  by  the  battery,  proceeds /rom  the  solid  to  the 
fluid  which  acts  upon  it  chemically.  Thus,  in  a  battery  composed  of  zinc, 
diluted  sulphuric  acid,  and  copper,  the  acid  acts  upon  the  zinc,  and  not  on 
the  copper.  The  galvanic  fluid  proceed,  therefore,  from  the  zinc  to  the 
acid,  from  the  acid  to  the  copper,  &c.  Instead  of  using  two  different 
metals  to  form  the  galvanic  circuit,  one  metal,  in  difterent  states,  may  be 

274.  What  is  the  voltaic  battery?  What  is  said  in  the  note  with  re- 
gard to  the  electricity  excited  by  the  battery  ?  What  does  Fig.  138  rep- 
resent ?    Of  what  does  the  voltaic  battery  consist  ? 


224  NATURAL  PHILOSOPHY. 

a  plate  of  zinc,  connected  with  the  copper  plate  of  the  suc- 
ceeding cell,  and  a  copper  plate  joined  with  the  zmc  plate  of 
the  or'ecedino-  cell.  These  pairs  must  commence  with  copper 
and  terminate  with  zinc,  or  commence  with  zinc  and  termmate 
with  copper.  The  communication  between  the  first  and  last 
plates  is  made  by  wires,  which  thus  complete  the  galvanic 
circuit.  The  substance  to  be  submitted  to  galvanic  action  is 
placed  between  the  points  of  the  two  wires.     _     ,    .    ,  . 

2  A  compound  battery  of  great  power  is  obtamed  by 
uniting  a  number  of  these  troughs.  In  a  similar  manner  a 
battery  may  be  produced  by  uniting  several  piles,  masmg  a 
metallic  communication  between  the  last  plate  oi  the  one  and 
the  first  plate  of  the  next,  and  so  on,  taking  care  that  the 
order  of  succession  of  the  plates  in  the  circuit  be  preserved 
inviolate. 

3.  The  Couronne  des  tasses,  repre-  Fig.  139. 

sented  in  Fig.  139,  is  another  form 
of  the  galvanic  battery.  It  consists 
of  a  number  of  cups,  bo  wis,  or  glasses, 
with  the  zinc  and  copper  plates  im- 
mersed in  them,  in  the  order  repre- 
sented in  the  figure  ;  Z  indicating 
the  zinc,  and  C  the  copper  plates ; 
the  arrows  denoting  the  course  of 
the  electiic  fluid. 

4  The  electric  shock  from  the  voltaic  battery  may  be  re- 
ceived  by  any  number  of  persons,  by  joining  hands,  havmg 
previously  wetted  them. 

emploved  ;^  the  essential  principle  being,  that  one  of  the  elements  shall  be 
more  powerfully  affected  by  some  chemical  agent  than  the  other  Ihus, 
if  a  aalvanic  pair  be  made  of  the  same  metal,  one  part  must  be  softer  than 
the  other,  (as  is  the  case  with  cast  and  rolled  zinc  ;)  or  a  greater  amount 
of  snrface  must  be  exposed  to  corrosion  on  one  side  than  on  the  other ;  or 
a  more  powerful  chemical  agent  be  used  on  one  side,  so  that  a  current  wid 
be  sent  from  the  part  most  corroded,  through  the  liquid,  to  the  pa)t  least 
corroded,  whenever  the  poles  are  united  and  the  circ.t  thereby  com- 

pleted.  — 

"  Ho^sThe  communication  between  the  first  and  last  plates  madel 
Where  must  the  substance,  which  is  to  be  submitted  to  galvanx  action, 
be  p-aced?  How  can  a  compound  battery  of  great  power  be  obtamed? 
Wiiat  does  FiR.  139  represent?  Of  what  does  this  battery  consist?  How 
can  the  electric  shock,  from  the  voltaic  battery,  be  received  by  any  num- 
ber  of  persons  ? 


GALVANISM. 


225 


Fig.  140. 


5.  Smee's  Galvanic  Battery  is  represented  in  Fig.  140, 
and  affords  an  instance  of  a  battery  in  its  simplest  form.  It 
consists  of  a  glass  vessel,  (as  a  tumbler,)  on 
which  rests  the  frame  that  supports  the  ap- 
paratus Avithin.  Two  screw-cups  rise  from 
the  fi-ame,  to  which  wires  may  be  attached 
for  the  conveyance  of  the  electric  current 
in  any  direction.  One  of  the  screw-cups 
communicates  with  a  thin  strip  of  platinum, 
or  platinum  foil,  which  is  suspended  with- 
in the  glass  vessel  between  two  plates  of 
zinc,  thus  presenting  each  surface  of  the 
platinum  to  a  surface  of  zinc ;  and  the  gal- 
vanic action  is  in  proportion  to  the  extent  of  the  opposite  ^r- 
faces  of  the  two  metals,  and  their  nearness  to  each  other.  The 
other  screw-cup  is  connected  with  the  two  zinc  plates.  The 
screw-cup  connected  with  the  platinum  is  insulated  from  the 
metalhc  frame  which  supports  it,  by  rosewood,  and  a  thumb- 
screw confines  the  zinc  plates,  so  that  they  can  be  renewed 
when  necessary.  The  liquid  employed  for  this  battery  is  sul- 
phuric acid,  or  oil  of  vitriol,  diluted  with  ten  parts  of  water  by 
measure.  To  prevent  the  action  of  the  acid  upon  the  zinc 
plates,  their  surfaces  are  commonly  amalgamated,  or  combined 
with  mercury,  which  prevents  any  chemical  action  of  the  acid 
with  the  zinc  until  the  galvanic  circuit  is  established,  when  the 
zinc  is  immediately  attacked  by  the  acid. 

6.  Fig.  141  represents  a  series  of  three  pairs  of  this  battery. 

Fig.  141. 


in  which  it  will  be  observed  that  the  platinum  of  one  is  connected 
with  the  zinc  of  the  next,  and  that  the  terminal  wires  proceed. 

What  figure  represents  Smee's  galvanic  battery?  Describe  it.  What 
liniiid  is  employed  for  this  battery  ?    Describe  Fig.  141. 

10* 


226 


NATURAL  PHILOSOPHY. 


consequently,  one  from  a  platinum  plate,  and  the  other  from 
a  zinc  plate,  as  in  a  single  pair. 

7.  Sulphate  of  Copper  Battery.  Fig.  142  represents  a 
sulphate  of  copper  battery,  and  Fig.  143  a  Yertical  section  of 
the  same  battery.  It  con- 
sists of  a  double  cyhnder 
of  copper,  C  C,  Fig.  143, 
vdrh  a  bottom  of  the  same 
metal,  which  sei'Yes  the 
double  purpose  of  a  gal- 
vanic plate,  and  a  vessel 
to  contain  the  exciting  so- 
lution. The  solution  is 
contained  in  the  space 
between  the  two  copper 
cylinders.  A  moveable 
cyhnder  of  zinc,  Z,  is  let 
down  into  the  solution 
whenever  the  battery  is 
to  be  used.  It  rests  on 
three  arms  of  wood  or 
ivory  at  the  top,  by  means 
of  which  it  is  insulated. 
Thus  suspended  in  the 
solution,  the  surfaces  of 
zinc  and  copper  respec- 
tively, face  each  other. 
A  screw- cup,  X,  is  at- 
tached to  the  zinc,  and 
an::her,  P,  to  the  copper 
cyhnder,  to  receive  the 
wi  es.  ^Yhen  a  commu- 
nication is  made  be- ween 

the  two  cups,  electricity  is  excited.  The  liquid  employed  in 
this  batt^iy  is  a  solution  of  sulphate  of  copper  (common  blue 
viijii)!)  in  water.  A  saturated  solution  is  first  made,  and  to 
this  solution  as  much  more  water  is  added. ^ 


Fis.  143. 


*  A  pint  of  water  will  dissolve  about  a  quarter  of  a  pound  of  b'ne  vitriol. 
The  solution  described  above  will  therefore  contain  about  two  ounces  of 
the  salt  to  the  pint.    The  addition  of  alcohn],  in  small  quantities,  increases 


Describe  the  sulphate  of  copper  battery. 


GALVANISM. 


227 


8.  Grove's  Battery.  This  is  the  most  energetic  battery- 
yet  known,  and  is  the  one  most  oenei-ally  used  for  the  mag- 
netic telegraph.  The  metals  ciuj)l()}  ed  iire  plaLinum  and  zinc, 
and  the  solutions  are  sti'ong  niliic  acid,  in  contact  with  the 
platinum,  and  sulphuric  acid  diluted  with  ten  or  twelve  parts  of 
water  in  contact  with  the  zinc.  This  battery  must  be  used 
with  great  care  on  account  of  the  strength  of  the  acids  used 
for  the  solutions,  which  send  out  injurious  fumes,  and  which 
are  destructive  to  organic  substances.  Fig.  144  represents 
Grove's  battery.  The  containing  vessel  is  glass  ;  within  this 
IS  a  thick  cyhnder  of  amalgamated  zinc, 
standing  on  short  legs,  and  divided  by 
a  longitudinal  opening  on  one  side,  in  order 
to  allow  the  acid  to  circulate  freely.  Inside 
of  this  is  a  porous  cell  of  unglazed  porcelain, 
containing  the  nitric  acid,  and  strip  of  plati- 
num. The  plciiinum  is  supported  by  a  strip 
of  brass  fixed  by  a  thumb-screw  and  an  in- 
sulating piece  of  ivory  to  the  arm  proceed- 
mg  from  the  zinc  cylinder.  The  amal- 
gamated zinc  is  not  acted  upon  by  the 
diluted  sulphuric  acid,  imtil  the  circuit  of  the  battery  is  com- 
pleted.   But  as  the  nitric  acid  will  filter  through  the  porous 

the  permanency  of  the  action  of  the  solution.  The  zinc  cyhnder  should  al- 
ways be  taken  out  of  the  solution  when  the  battery  is  not  in  use  ;  but  the 
solution  may  remain  in  the  battery.  The  battery  will  keep  in  good  action 
for  twenty  or  thirty  minutes  at  a  time. 

The  sulphate  of  copper  battery,  although  not  so  energetic  as  Smee's,  is 
found  very  convenient  in  a  large  class  of  experiments,  and  is  particularly 
recommended  to  those  who  are  inexpert  in  the  i:se  of  acids ;  because  the 
sulphate  of  copper  being  entirely  neutral,  will  not  injure  the  color  nor  the 
texture  of  organic  substances. 

There  is  another  form  of  the  sulphate  of  copper  battery,  called  the  Pi  o- 
tected  Sulphate  of  Copper  Battery,  which  differs  from  the  one  described 
in  having  a  porous  cell  of  earthenware,  or  leather,  interposed  between  the 
zinc  and  the  copper,  thus  forming  two  cells,  in  the  outer  of  which  sulphate 
of  copper  may  be  used,  and  in  the  inner  one  a  solution  of  sulphate  of 
soda,  (Glauber  salt,)  or  chloride  of  sodium,  (common  salt,)  or  even  dilute 
sulphuric  acid.  This  battery  will  continue  in  use  for  several  days,  and  it 
is  therefore  of  great  use  in  the  electrotype  process. 


Describe  Grove's  battery.  Which  is  the  most  powerful  battery  ?  For 
what  is  it  used  ? 


228 


NATURAL  PHILOSOrHY. 


cell,  and  act  upon  the  zinc,  it  is  advisable  to  remove  the  zinc 
from  the  acid  when  the  battery  is  to  remain  inactive.  The 
action  of  Grove's  battery  may  be  considered  as  three  times 
greater  than  that  of  the  sulphate  of  copper  battery. 

275.  The  spark  from  a  powerful  voltaic  battery  acts 
upon  and  inflames  gunpowder,  charcoal,  cotton,  and 
other  inflammable  bodies,  fuses  all  metals,  burns  up  or 
disperses  diamonds  and  other  substances  on  which  heat 
in  other  forms  produces  little  or  no  effect.* 

The  wires,  by  which  the  circuit  of  the  battery  is  completed, 
are  generally  covered  with  glass  tubes,  in  order  that  they  may 
be  held,  or  directed  to  any  substance. 

276.  There  are  three  principal  circumstances  in  which 
the  electricity  produced  by  the  galvanic  or  voltaic  bat- 
tery, differs  from  that  obtained  by  the  ordinary  electrical 
machine,  namely, — 

1st.  The  very  low  degree  of  intensity \  of  that  pro- 

*  The  most  striking  effects  of  Galvanism  on  the  human  frame,  after 
death,  were  exhibited  at  Glasgow,  a  few  years  ago.  The  subject  on  which 
the  experiments  were  made,  was  the  body  of  the  murderer  Clydesdale, 
who  was  hanged  at  that  city.  He  had  been  suspended  an  hour,  and  the 
first  experiment  was  made  in  about  ten  minutes  after  he  was  cut  down. 
The  galvanic  battery  employed  consisted  of  270  pairs  of  four-iuch  plates. 
On  the  application  of  the  battery  to  different  parts  of  the  body,  every 
muscle  was  thrown  into  violent  agitation ;  the  leg  was  thrown  out  with 
great  violence,  breathing  commenced,  the  face  exhibited  extraordinary 
grimaces,  and  the  finger  seemed  to  point  out  the  spectators.  Many  per- 
sons were  obliged  to  leave  the  room  from  terror  or  sickness  ;  one  gentle- 
man fainted,  and  some  thought  that  the  body  had  really  come  to  life. 

t  By  intensity  is  here  meant  the  same  that  is  impHed  by  density,  as 
applied  to  matter.  The  quantity  of  electricity  obtained  by  galvanic  action 
is  much  greater  than  can  be  obtained  by  the  machine ;  but  it  flows,  as  it 
were,  in  narrow  streams.    The  action  of  the  electrical  machine  may  be 


275  What  effects  may  be  produced  by  a  spark  from  a  voltaic  battery? 
What  precaution  is  taken  in  regard  to  the  lines  of  the  circuit  ? 

276.  In  how  many  ways  does  the  electricity  produced  by  the  galvanic 
or  voltaic  battery  differ  from  that  obtained  by  the  ordinary  electrical  ma- 
chine? What  is  the  first?  What  is  here  meant  by  intensity?  How  does 
the  quantity  of  electricity  obtained  by  galvanic  action,  compare  with  that 
obtained  by  the  machine  ? 


GALVANISM. 


229 


duced  by  the  i^alvanic  battery,  compared  with  that  ob- 
tained by  the  machine. 

2d.  The  very  large  quantity  of  electricity  which  is 
set  in  motion  by  the  voltaic  battery  ;  and, 

3d.  The  continuity  of  the  current  of  voltaic  electricity, 
and  its  perpetual  reproduction,  even  while  this  current 
is  tending  to  restore  the  equilibrium.* 

A  common  electrical  battery  may  be  charged  from  a  voltaic 
battery  of  sufficient  size  ;  but  a  battery  constructed  of  a  small 
number  of  pairs,  even  though  the  plates  are  large,  furnishes  no 
indication  of  attraction  or  repulsion  equal  to  that  whicli  is  given 

compared  to  a  mighty  torrent,  dashing  and  exhausting  itself  in  one  leap 
from  a  precipitous  height.  The  galvanic  action  may  be  compared  to  a 
steady  stream,  supplied  by  an  inexhaustible  fountain.  In  other  words,  the 
monientum  of  the  electricity  excited  by  galvanism  is  less  than  that  from 
the  electrical  machine  ;  but  the  quantity,  as  has  been  stated,  is  greater. 

*  Whenever  an  electrical  battery  is  charged,  how  great  soever  may  be 
the  quantity  that  it  contains,  the  whole  of  the  power  is  at  once  expended, 
as  soon  as  the  circuit  is  completed.  Its  action  may  be  sufficiently  ener- 
getic while  it  lasts,  but  it  is  exerted  only  for  an  instant,  and  like  the  de- 
structive operation  of  lightning,  can  effect,  during  its  momentary  passage, 
only  sudden  and  violent  changes,  which  it  is  beyond  human  power  to  reg- 
ulate or  control.  On  the  contrary,  the  voltaic  battery  continues,  for  an 
indefinite  time,  to  develop  and  supply  vast  quantities  of  electricity,  which, 
far  from  being  lost  by  returning  to  their  source,  circulate  in  a  perpetual 
stream,  and  w^ith  undiminished  force.  The  effects  of  this  continued  cur- 
rent on  the  bodies  subjected  to  its  action,  will,  therefore,  be  more  definite, 
and  will  be  constantly  accumulating ;  and  their  amount,  in  process  of 
Ime,  will  be  incomparably  greater  than  even  those  of  the  ordinary  elec- 
trical explosion.  It  is  therefore  found  that  changes  in  the  composition  of 
bodies  are  effected  by  galvanism,  w^hich  can  be  accomplished  by  no  other 
means.  The  science  of  Galvanism,  therefore,  has  extended  the  field,  and 
multiplied  the  means  of  investigation  in  the  kindred  sciences,  especially 
that  of  Chemistry. 


To  what  may  the  action  of  the  electrical  machine  be  compared  ?  To 
what  may  the  galvanic  action  be  compared?  What  is  the  second  way  in 
which  they  differ?  What  is  the  third  ?  What  is  said  in  the  note  with 
regard  to  the  third  circumstance  in  which  the  electricity  obtained  by  the 
ordinary  electrical  machine  differs  from  that  produced  by  the  galvanic 
battery?  What  is  said  of  the  effects  of  this  contmued  current  on  the  bodies 
subjected  to  its  action  ? 


230 


NATURAL  PHILOSOPHY. 


by  the  feeblest  degree  of  excitation  to  a  piece  of  sealing-wax. 
A  galvanic  battery,  consisting  of  fifty  pairs  of  plates,  will  affect 
a  delicate  goldleaf  electrometer;  and,  with  a  series  of  one 
thousand  pairs,  even  piih  balls  are  made  to  diverge. 

277.  The  effect  of  the  voltaic  pile  on  the  animal 
body  depends  chiefly  on  the  number  of  plates  that  are 
employed;  but  the  intensity  of  the  spark  and  its  chemi- 
cal agencies  increase  more  with  the  size  of  the  plates, 
than  with  their  number.  Galvanism  explains  many 
facts  in  common  life. 

1.  Porter,  ale,  or  strong  beer,  is  said  to  have  a  peculiar  taste 
when  drunk  from  a  pewter  vessel.  The  peculiaiity  of  taste  is 
caused  by  the  galvanic  circle  formed  by  the  pewter,  the  beer, 
(tc,  and  the  moisture  of  the  under  lip. 

Works  of  metals,  the  parts  of  which  are  soldered  together, 
soon  tarnish  in  the  places  where  the  metals  are  joined. 

Ancient  coins,  composed  of  a  mixture  of  metal,  have  crum- 
bled  to  pieces,  while  those  composed  of  pure  metal  have  been 
uninjured. 

The  nails  and  the  copper  in  sheathing  of  ships  are  soon 
corroded  about  the  place  of  contact.  These  are  all  the  effects 
of  galvanism. 

There  are  persons  who  profess  to  be  able  to  find  out  seams 
in  brass  and  copper  vessels  by  the  tongue,  which  the  eye 
cannot  discover ;  and,  by  the  same  means,  to  distinguish  the 
base  mixtures  which  abound  in  gold  and  silver  trinkets. 

2.  From  what  has  now  been  stated,  it  will  be  seen  that  the 
efi:ects  of  galvanic  action  depend  on  two  circumstances,  namely  ; 
1st,  the  size  of  the  plates  employed  in  the  circuit ;  and,  2dly, 
the  number  of  the  pairs  constituting  a  battery.  But  there  is 
a  remarkable  circumstance  to  be  noticed  in  tliis  connexion  ; 
namely,  that  thei  e  is  one  class  of  facts  dependent  on  the  ex- 
tension of  the  size  of  the  plates,  and  another  on  the  increase  of 
theii'  number.  TJie  power  to  develop  heat  and  magnetism  is  de- 
pendent on  the  size  of  the  plates,  that  is,  on  the  extent  of  the 
surfaces  acted  upon  by  the  chemical  agent ;  while  the  power  to 
decompose  chemical  compounds,  and  to  affect  the  animal  sys- 
tem, is  affected  in  a  greater  raiio  by  the  increase  of  tlie  number 


277.  On  what  does  the  effect  of  the  voltaic  pile  on  the  body  depend  ? 
What  facts  in  common  life  does  galvanism  explain?  On  what  does  tho 
effects  of  galvanic  action  depend  ? 


GAl.VANIdM. 


231 


of  the  pairs.  Balteiies  construcicd  of  large  plates  are  some- 
times called  Caloruaofors,'^  from  their  gre:it  power  of  pro- 
ducing heat.  They  usually  consist  of  fiom  one  to  eight  pairs 
of  plates.  They  are  made  in  various  forms ;  sometimes  the 
sheets  of  copper  and  zinc  are  coiled  in  concentric  spirals, 
sometimes  placed  side  by  side;  and  they  may  be  divided  into 
a  great  number  of  small  plates,  provided  that  all  the  zinc  plates 
are  connected  together,  and  all  the  copper  plates  together,  and  then 
that  the  experiments  are  performed  in  a  channel  of  communication^ 
opened  betiveen  the  sets  of  plates,  and  not  between,  pairs  as  in 
the  common  battery,  for  it  is  immateiial  whether  one  large  sur- 
face be  used,  or  many  small  ones  electiically  connected  to- 
gether. The  effect  of  all  these  arrangements,  by  Avhich  the 
metallic  surface  of  a  single  pair  is  augmented,  is  to  increase  the 
quantity  produced. 

3.  The  galvanic  or  voltaic  battery  is  one  of  the  most  valua- 
ble acquisitions  of  modern  science.  It  has  proved  m  many  in- 
stances the  key  by  which  science  has  entered  into  the  innermost 
recesses  of  nature,  and  discovered  the  secret  of  many  of  her 
operations.  It  has,  in  great  measure,  lifted  the  hitherto  im- 
penetrable veil  that  has  concealed  the  mysterious  workings  in 
the  material  world,  and  has  opened  a  field  for  investigation 
and  discovery  as  inviting  as  it  is  boundless.  It  has  strengthen- 
ed the  sight,  and  enlarged  the  view  of  the  philosopher  and 
the  man  of  science,  and  given  a  degree  of  certainty  to  scien- 
tific inquiry  hitherto  known  to  be  unreached,  and  supposed  to 
be  unattainable  ;  and  if  it  has  not  yet  satisfied  the  hopes  of  the 
Alchymist,  nor  emulated  the  gold- converting  touch  of  Midas, 
it  has  shown,  almost  to  demonstration,  that  science  may  yet 
achieve  wonders  beyond  the  stories  of  Mythology,  and  realize 
the  familiar  adage  that'"  truth  is  stranger  than  fiction.^ ^ 

*  The  name  Calorimotor  (that  is,  the  mover  of  heat)  was  applied  by 
Dr.  Hare  of  Philadelphia  to  a  very  powerful  apparatus  which  he  construct- 
ed, and  which  he  found  possessed  of  a  very  remarkable  power  in  producing 
heat. 


What  are  batteries  constructed  of  large  plates  sometimes  called  ?  Why  1 
Describe  them.  Upon  what  principle  is  the  calorimotor  constructed  ?  Are 
galvanic  batteries  of  value  1 


232 


NATURAL  PHILOSOPHY 


CHAPTER  Xni. 

MAGNETISM  AND  ELECTRO-MAGNETISM. 

278.  Magnetism  treats  of  the  properties  and  effects 
of  the  magnet,  or  loadstone.* 

There  are  two  kinds  of  magnets,  namely,  the  native 
or  natural  magnet,  and  the  artificial. 

The  native  magnet,  or  loadstone,!  is  an  ore  of  iron, 
found  in  iron  mines,  and  has  the  property  of  attracting 
iron  and  other  substances  which  contain  it. 

A  permanent  artificial  magnet  is  a  piece  of  iron  to 
which  permanent  magnetic  properties  have  been  com- 
municated. 

For  all  purposes  of  accurate  experiment,  the  artificial 
is  to  be  preferred  to  the  native  magnet. 

If  a  straight  bar  of  soft  iron  be  held  in  a  vertical  posiiion, 
(or,  still  better,  in  a  position  slightly  inclined  to  the  perpen- 
diculai-,  the  lower  end  deviating  to  the  north,)  and  struck 
several  smait  blows  with  a  hammer,  it  will  be  found  to  Ivdxe 

*  That  part  of  science  which  relates  to  the  development  of  ina^rnetism 
by  means  of  a  current  of  electricity,  will  be  noticed  under  the  head  of 
Electro-Magnetism,  in  which  connexion  will  also  be  mentioned  the  de- 
velopment of  electricity  by  magnetism,  to  which  the  term  Magneto-Elec- 
tricity has  been  applied. 

t  Certain  ores  of  iron  are  found  to  be  naturally  possessed  of  magnetic 
properties,  and  are  therefore  called  natural  or  native  magnets,  or  load- 
stones. Besides  iron  and  some  of  its  compounds,  nickel  and  perhaps  cobalt 
also  possess  magnetic  propert.ey.  But  all  conductors  of  electricity  are  ca- 
pable of  exerting  the  m;.gnetic  properties  of  attraction  and  repulsion  while 
conveying  a  current  of  electricity,  as  will  be  shown  under  the  head  of 
Electro-Magnetism. 


278.  Of  what  does  Magnetism  treat?  How  many  kinds  of  magnets 
are  there?  W^hat  are  they  ?  What  is  the  native  magnet?  What  prop- 
erty does  it  possess?  W'iiat  is  an  artificial  magnet?  What  magnet  is 
preferred,  for  all  purposes  of  accurate  experiment  ?  How  can  an  artuioial 
magnet  be  mads? 


MAGNETISM. 


233 


acquired,  by  tliis  process,  all  the  properties  of  a  magnet;  or, 
in  other  words,  it  will  become  an  artificial  magnet. 

"219.  The  properties  of  a  magnet  are  four  ;  namely, 

1.  Polarity. 

2.  Attraction  of  unmagnetic  iron. 

3.  Attraction  and  repulsion  of  magnetic  iron. 

4.  The  power  of  communicating  magnetism  to  other  iron. 
Besides  these  properties  the  magnet  has  recently  been  dis- 

covej-ed  to  be  possessed  of  electrical  properties.  These  will  be 
considered  in  another  connexion. 

280.  By  the  polarity  of  a  magnet  is  meant  the  prop- 
erty of  pointing,  or  turning  to  the  north  and  south  poles. 
The  end  which  points  to  the  north,  is  called  the  north 
pole  of  the  magnet,  and.  the  other  the  south  pole.  The 
attractive  power  of  a  magnet  is  strongest  at  the  poles.* 

When  a  magnet  is  supported  in  such  a  manner  as  to 
move  freely,  it  will  spontaneously  assume  a  position 
directed  nearly  north  and  south. f 

There  are  several  ways  of  supporting  a  magnet,  so  as  to 
enable  it  to  manifest  its  polarity.  First,  by  suspending  it, 
accurately  balanced,  from  a  string.  Secondly,  by  poising  it  on 
a  sharp  point.  Thirdly,  by  attaching  it  to  some  buoyant  sub- 
stance and  allowing  it  to  float  freely  on  water. 

*  The  attractive  power  of  a  magnet  is  generally  stated  to  be  greatest 
at  the  poles;  but  the  actual  poles,  or  pohits  of  greatest  magnetic  intensity 
in  a  steel  magnet,  are  not  exactly  at  the  ends,  but  a  little  within  them. 

t  The  points  to  which  the  poles  of  a  magnet  turn  are  tlie  magnetic 
poles.  These  do  not  exactly  coincide  with  the  astronomical  poles  of  the 
earth  ;  but  although  the  value  of  the  magnetic  needle  has  been  predicated 
on  the  supposition  that  its  polarity  is  a  tendency  to  point  exactly  to  the 
north  and  south  poles  of  the  earth,  the  recent  discovery  of  the  magnetic 
poles,  as  the  points  of  attraction,  has  not  depreciated  the  value  of  the  com- 
pass, because  the  variation  is  known  and  can  be  corrected. 


279.  What  is  the  first  property  of  the  magnet?  Second?  Third? 
Fourth  ? 

280.  What  is  meant  by  the  polarity  of  a  magnet?  Where  is  the  at- 
tractive power  of  a  magnet  the  strongest  ?  When  will  a  magnet  assume 
a  position  directed  nearly  north  or  south  ?  What  is  the  north  pole  of  the 
magnet?  What  is  the  south  pole  ?  In  what  ways  can  a-magnet  be  sup- 
ported so  as  to  enable  it  to  manifest  its  polarity  ? 


234 


NATURAL  PHILOSOPHY. 


281.  Different  poles  of  magnets  attract,  and  similar  . 
poles  repel  each  other.* 

1.  A  magnet,  whether  native  or  artificial,  attracts  iron  or 
steel  which  has  no  magnetic  properties ;  but  it  both  attracts 
and  repels  those  substances,  when  they  are  magnetic ;  that  is, 
the  north  pole  of  one  magnet  will  attract  the"^  south  pole  of 
another,  and  the  south  pole  of  one  will  attract  the  north  of 
another ;  but  the  north  pole  of  the  one  repels  the  north  pole  of 
the  other,  and  the  south  pole  of  one  repels  the  south  pole 
of  another. 

^  ?.  If  either  pole  of  a  magnet  be  broiio-]it  near  anv  small 
piece  of  soft  iron,  it  will  attract  it.  Iron  fihngs  will  also'adhere 
in  clusters  to  either  pole. 

282.  A  magnet  may  communicate  its  properties  to 
other  unmagnetized  bodies.  But  these  properties  can 
be  conveyed  to  no  other  substances  than  iron,  nickel,  or 
cobalt,  without  the  aid  of  electricity.! 

A\\  permanent  natural  and  artificial  magnets,  as  well 
as  the  bodies  on  which  they  act,  are  either  iron  in  its 
pure  state,  or  such  compounds  as  contain  it. 

The  powers  of  a  magnet  are  increased  bv  action,  and 
are  impaired  and  even  lost  by  long  disuse. ' 

*  There  is  here  a  close  analog-y  between  the  attractive  and  repulsive 
powers  of  the  positive  and  the  negative  forms  of  electricity,  and  the  north- 
ern and  southern  polarities  of  the  magnet.  The  same  law  obtains  with 
regard  to  both,  namely— ^efiree/?  like  powers  there  is  repulsion;  between 
unlike,  there  is  attraction. 

t  The  accuracy  of  the  above  statement  may,  perhaps,  be  questioned, 
since  Coulomb  has  discovered,  that  all  solid  bodies  are  susceptible  of 
magnetic  influence:'  But  the  influence''  is  perceptible  oniv  by  the 
nicest  tests,  and  under  peculiar  circumstances.    [See  Eiectro-Ma'^netism  ] 

281.  How  do  the  same  and  diiFereut  poies  of  a  maofuet  aitect  each  other? 
What  is  sajd  with  regard  to  the  attraction  of  magnets,  whether  native  cr 
artificial  ?  What  analogy  is  there  between  the  attractive  and  repulsive 
powers  of  the  different  kinds  of  electricity,  and  the  northern  and  southern 
polarities  of  the  magnet? 

282.  Can  a  magnet  communicate  its  properties  to  other  bodies?  To 
what  substances,  only,  can  these  properties  be  conveyed  ?  Of  what  sub 
stances  are  all  natural  and  artificial  magnets,  as  well  as  the  bodies  on 
which  they  act,  composed  ?  How  can  the  powers  of  a  magnet  be  in- 
creased ? 


MAGNETISM. 


235 


1.  When  the  two  poles  of  a  magnet  are  brouglit  together, 
so  that  the  magnet  resembles  in  shape  a  horse-shoe,  or  the 
capital  letter  U,  it  is  called  a  horse-shoe  magnet,  or  a  U  mag- 
net, and  it  may  be  made  to  sustain  a  considerable  weiglit  by 
suspending  substances  from  a  small  iron  bar,  extending  from 
one  pole  to  the  other.  This  bar  is  called  the  keeper.  A  small 
addition  may  be  made  to  the  weight  every  day. 

2.  Soft  iron  acquires  the  magnetic  power  very  readily,  and 
also  loses  it  as  readily, — hardened  iron  or  steel  acquires  the 
property  with  difficulty,  but  retains  it  permanently. 

283.  When  a  magnet  is  broken  or  divided,  each  part 
becomes  a  perfect  magnet,  having  both  a  north  and 
south  pole. 

This  is  a  remarkable  circumstance,  since  the  central  part  of 
a  magnet  appears  to  possess  but  little  of  the  magnetic  power, 
— but  when  a  magnet  is  divided  in  the  centre,  this  very  part 
assumes  the  magnetic  power,  and  becomes  possessed,  in  the 
one  part,  of  the  north,  and  in  the  other,  of  the  south  polarity. 

284.  The  magnetic  power  of  iron  or  steel  resides 
wholly  on  the  surface,  and  is  independent  of  its  mass.^ 

*  111  this  respect  there  is  a  strong  resemblance  between  magnetism  and 
electricity.  Electricity,  as  has  already  been  stated,  is  wholly  confined  to 
the  surface  of  bodies.  In  a  few  words,  magnetism  and  electricity  may  be 
said  to  resemble  each  other  in  the  following  particulars : 

1.  Each  consists  of  two  species,  namely,  the  vitreous  and  the  resinous 
(or,  the  positive  and  negative)  electricities  ;  and  the  northern  and  southern 
(sometimes  called  the  Boreal  and  the  Astral)  polarity. 

2.  In  both  magnetism  and  electricity,  those  of  the  same  name  repel,  and 
those  of  dilierent  names  attract  each  other. 

3.  The  laws  of  induction  in  both  are  similar. 

4.  The  influence,  in  both  cases,  (as  has  just  been  stated,)  resides  at  the 
surface,  and  is  wholly  independent  of  their  mass. 

What  is  a  horse-shoe  magnet?  How  can  it  be  made  to  sustain  a  con- 
siderable weight  ?  What  is  this  bar  called  ?  How  does  soft  iron  differ 
from  hardened  iron,  with  respect  to  its  acquiring  and  losing  the  magnetic 
power  ? 

283.  Wiiat  effect  is  produced  when  a  magnet  is  broken  or  divided? 
Why  is  this  a  remarkable  circumstance  ? 

284.  Where  does  the  magnetic  power  of  iron  or  steel  wholly  reside  ? 
Note.  In  what  particulars  do  magnetism  and  electricity  resemble  each 
other?  What  is  the  first?  What  is  the  second?  What  is  the  third'? 
What  is  the  fourth? 


236 


NATURAL  PHILOSOPHY. 


2S5.  Heat  weakens,  and  a  great  deorree  of  heat  de- 
stroys the  power  of  a  magnet ;  but  the  magnetic  attrac- 
tion is  undiminished  by  the  interposition  of  any  bodies, 
except  iron,  steel,  &:c.  * 

Electricity  frequently  changes  the  poles  of  a  magnet ;  and 
the  explosion  of  a  small  quantity  of  gunpowder,  on  one  of  the 
poles,  produces  the  same  effect. 

Electricity,  also,  sometimes  renders  iron  and  steel  mao-netic, 
which  were  not  so  before  the  charge  was  received.  ^ 

286.  The  effect  produced  by  two  magnets,  used  to- 
gether, are  much  more  than  double  that  of  either  one 
used  alone. 

hen  a  magnet  is  suspended  freely  from  its  centre, 
the  two  poles  will  not  lie  in  the  same* horizontal  direc- 
tion. This  is  called  the  inclination  or  the  dipjnns:  of 
the  magnet.* 

287.  The  magnet,  when  suspended,  does  not  in- 
yariably  point  exactly  to  the  north  and  south  points, 
but  yaries  a  little  towards  the  east  or  the  west.  This 
yariation  differs  at  different  places,  at  different  seasons, 
and  at  different  times  in  the  day.y 

*  The  teudency  of  a  magnetic  needle  to  dip  is  corrected  in  the  mari 
ner s  and  surveyors  compasses,  by  making  the  south  ends  of  the  needles  in- 
tended for  use  in  northern  latitudes  somewhat  heavier  than  the  north  ends. 
Compass-needles,  intended  to  be  employed  on  long  voyages  where  great 
variations  of  latitude  may  be  expected,  are  furnished  with  a  small  sliding 
weight,  by  the  adjusting  of  which  the  tendency  to  dip  may  be  countert 
acted.  The  cause  of  the  dipping  of  the  needle  is  the  superior  attraction 
caused  by  the  closer  proximity  of  the  pole  of  the  magnet  to  the  magnetic 
pole  of  the  earth.  In  north  latitude,  the  north  pole  of  the  needle  dips  :  in 
south  latitude,  the  south  pole. 

t  The  variation  of  the  magnetic  needle  from  what  has  been  supposed 
its  true  polarity,  was  a  phenomenon  that  for  centuries  had  baffled  the 


285.  What  effect  has  heat  on  the  power  of  the  magnet  ?  Bv  what  is 
the  magnetic  attraction  diminished  ?  What  effect  has  electricity  on  the 
poles  of  a  magnet  ?    \yhat  effect  has  electricity  sometimes  on  iron  aiid  steel  ? 

2^6.  What  proportion  do  the  effects  produced  by  two  maanets,  used  to- 
gether, bear  to  that  of  either,  used  alone  ?  What  is  meant  hy  the  inclina- 
tion or  dippinof  of  the  magnet  ? 

287.  Does  the  magnet,  when  suspended,  invariably  point  to  the  north 
and  south  points  ? 


MAGNETISM. 


237 


I.  The  science  of  magnetism  has  rendered  immense  advan- 
tages to  commerce  and  navigation,  by  means  of  the  mariner's 
compass.'*  The  mariner's  compass  consists  of  a  magnetized  bar 
of  steel,  called  a  needle  ;  having  at  its  centre  a  cap  fitted  to  it, 

science  of  the  philosopher  to  explain.  Recent  discoveries  have  given  a 
satisfactory  explanation  of  this  apparent  anomaly.  The  earth  has,  in  fact, 
four  magnetic  poles,  two  of  which  are  strong  and  two  are  weak.  The 
strongest  north  pole  is  in  America,  the  weakest  in  Asia.  The  earth  itself 
is  considered  as  a  magnet,  or  rather,  as  composed  in  part  of  magnetic  sub- 
stances, so  that  its  action  at  the  surface  is  irregular.  The  variation  of  the 
needle  from  the  true  geographical  meridian  is  therefore  subject  to  changes 
more  or  less  irregular. 

This  subject  is  very  ably  treated  in  "  Davis'  Manual  of  Magnetism," 
(edition  of  1847,)  to  which  the  student  is  referred,  as  probably  the  best 
treatise  on  the  subject  that  has  ever  been  published.  Mr.  Davis  is  one  of 
those  scientific  and  skilful  mechanics,  (of  whom  there  are  not  a  few  among 
us,)  who  have,  as  it  were,  forced  their  way  into  the  temple  of  science  amid 
discouragements  and  difficulties,  but  have  deposited  richer  gifts  on  the 
altar  than  most  of  those  whose  contributions  were  expected.  He  has 
originated  many  improvements  in  this  department  of  science  ;  and  his  de- 
votion to  the  subject  has  probably  rendered  him  as  familiar  with  all  the 
peculiar  phenomena  relating  to  it,  as  any  one  in  or  out  of  the  country. 
His  address  is,  Daniel  Davis,  Jr.,  Magnetical  Instrument  Maker,  428 
Washington-street,  Boston. 

*  The  invention  of  the  mariner's  compass  is  usually  ascribed  to  Flavio 
de  Melfi,  or  Flavio  Gioia,  a  Neapolitan,  about  the  year  1302.  Some  au- 
thorities, however,  assert  that  it  was  brought  from  China  by  Marco  Paolo, 
a  Venetian,  in  1260.  The  invention  is  also  claimed  both  by  the  French 
and  English. 

The  value  of  this  discovery  may  be  estimated  from  the  consideration, 
that,  before  the  use  of  the  compass,  mariners  seldom  trusted  themselves 
out  of  sight  of  land  ;  they  were  unable  to  make  long  or  distant  voyages,  as 
they  had  no  means  to  find  their  way  back.  This  discovery  enabled  them 
to  find  a  way  where  all  is  trackless  ;  to  conduct  their  vessels  through  the 
mighty  ocean,  out  of  the  sight  of  land  ;  and  to  prosecute  those  discoveries, 
and  perform  those  gallant  deeds,  which  have  immortalized  the  names  of 
Cook,  of  La  Perouse,  Vancouver,  Sir  Francis  Drake,  Nelson,  Parry, 
Franklin,  and  others. 


What  advantage  has  the  science  of  magnetism  rendered  to  commerce 
and  navigation?  Of  what  does  the  mariner's  compass  consist?  Note.  To 
whom  is  the  invention  of  the  mariner's  compass  usually  ascribed  ?  How 
may  the  value  of  this  discovery  be  estimated? 


238 


NATURAL  PHILOSOPHY. 


which  is  supported  on  a  sharp-pointed  pivot  fixed  in  the  base 
of  the  instrument.  A  circular  plate,  or  card,  the  circumference 
of  which  is  divided  into  degrees,  is  attached  to  the  needle,  and 
turns  with  it.  On  an  inner  circle  of  the  card  the  thirty-two 
points  of  the  mariner's  compass  are  inscribed.^ 

2.  The  needle  is  generally  placed  under  the  card  of  a  mari- 
ner's compass,  so  that  it  is  out  of  sight;  but  small  needles, 
used  on  land,  are  placed  above  the  card,  not  attached  to  it, 
and  the  card  is  permanently  fixed  to  the  box. 

288.  The  north  pole  of  a  magnet  is  more  powerful  in 
the  northern  hemisphere,  or  north  of  the  equator,  and 
the  south  pole  in  the  southern  parts  of  the  world. 

1.  When  a  piece  of  iron  is  brought  sufficiently  near  to  a 
magnet,  it  becomes  itself  a  magnet ;  and  bars  of  iron,  that 
have  stood  long  in  a  perpendicular  situation,  are  generally 
found  to  be  magnetical. 

2.  Artificial  magnets  are  made  by  applying  one  or  more 
powerful  magnets  to  pieces  of  soft  iron.f  "The  end  which  is 
touched  by  the  north  pole  becomes  the  south  pole  of  the  new 
magnet,  and  that  touched  by  the  south  pole  becomes  the 
north  pole.  The  magnet  which  is  employed  in  magnetizing  a 
steel  bar  loses  none  of  its  power  by  being  thus  employed  ;  and 
as  the  effect  is  increased  when  twv)  or  more  magnets  are  used, 
with  one  magnet  a  numbei"  of  bars  may  be  magnetized,  and 
then  combined  together ;  by  which  means  their  power  may  be 

*  The  compass  is  generally  fitted  by  two  sets  of  axes  to  an  outer  box, 
so  that  it  always  retains  a  horizontal  position,  even  when  the  vessel  rolls. 
When  the  artificial  magnet  or  needle  is  kept  thus  freely  suspended,  so 
that  it  may  t.  north  or  south,  the  pilot,  by  looking  at  its  position,  can 
ascertain  in  what  direction  his  vessel  is  proceeding ;  and,  although  the 
needle  varies  a  little  from  a  correct  polarity,  yet  this  variation  is  neither 
so  great,  nor  so  irregular,  as  seriously  to  impair  its  use  as  a  guide  to  the 
vessel  in  its  course  over  the  pathless  deep. 

t  This  mode  of  making  artificial  magnets  is  likely  to  be  wholly  super- 
seded by  the  new  mode  by  electrical  aid,  which  will  be  noticed  in  con- 
nexion with  Electro-Magnetism. 


288.  Where  are  the  north  and  south  poles  of  a  magnet  the  most  power- 
ful? What  etfect  has  a  magnet  on  a  piece  of  iron,  when  it  is  brought 
sufficiently  near  to  it?  How  are  artificial  magnets  made ?  Does  the  mao-- 
net  which  is  employed  in  magnetizing  a  steel  bar  lose  any  of  its  power  by 
being  thus  employed  ? 


MAGNETISM. 


289 


indefinitely  increased.  Sucli  an  apparatus  is  called  a  magnetic 
mar/a  zinc. 

3.  A  magnetic  needle  is  made  by  fastening  the  steel  on  a 
piece  of  board,  and  drawing  magnets  over  it  from  the  centre 
outwards. 

4.  A  horse-shoe  magnet  should  be  kept  armed,  by  a  small 
piece  of  iron  or  steel,  connecting  the  two  poles. 

5.  Interesting  experiments  may  be  made  by  a  magnet,  even 
of  no  great  power,  with  steel  or  iron  filings,  small  needles, 
pieces  of  ferruginous  substances,  and  black  sand,  which  contains 
iron.  Such  substances  may  be  made  to  assume  a  variety  of 
amusing  forms  and  positions,  by  moving  the  magnet  under  the 
card,  paper,  or  table,  on  which  they  are  placed.  Toys,  rep- 
resenting fishes,  frogs,  &c.,  which  are  made  to  appear  to  bite 
at  a  hook,  birds  floating  on  the  water,  &c.,  are  constructed  on 
magnetic  principles,  and  sold  in  the  shops. 

*  There  are  many  methods  of  makmg  artificial  magnets.  One  of  the 
most  simple  and  effectual  consists  in  passing  a  strong  horse-shoe  magnet 
over  bars  of  soft  iron. 

In  making  bar  (or  straight)  magnets,  the  bars  must  be  laid  lengthwise, 
on  a  flat  table,  with  the  marked  end  of  one  bar  against  the  unmarked  end 
of  the  next ;  and  in  making  horse-shoe  magnets,  the  pieces  of  steel,  pre- 
viously bent  into  their  proper  form,  must  be  laid  with  their  ends  in  con- 
tact, so  as  to  form  a  figure  like  two  capital  U's,  with  their  tops  joined  to- 
gether, thus  ,  observing  that  the  marked  ends  come  opposite  to  those 
which  are  not  marked  ;  and  then,  in  either  case,  a  strong  horse-shoe  mag- 
net is  to  be  passed,  with  moderate  pressure,  over  the  bars  ;  taking  care  to 
let  the  marked  end  of  this  magnet  precede,  and  its  unmarked  end  follow 
it ;  and  to  move  it  constantly  over  the  steel  bars,  so  as  to  enter  or  com- 
mence the  process  at  a  mark,  and  then  to  proceed  to  an  unmarked  end, 
and  enter  the  next  bar  at  its  marked  end,  and  so  proceed. 

After  having  thus  passed  over  the  bars  ten  or  a  dozen  times  on  each 
side,  and  in  the  same  direction,  as  to  the  marks,  they  will  be  converted 
into  tolerably  strong  and  permanent  magnets.  But  if,  after  having  con- 
tinued the  process  for  some  time,  the  exciting  magnet  be  moved  even  once 
over  the  bars  in  a  contrary  direction,  or  if  its  south  pole  should  be  permit- 
ted to  precede,  after  the  north  pole  has  been  first  used,  all  the  previously 
excited  magnetism  will  disappear,  and  the  bars  will  be  found  in  their 
original  state. 


What  is  a  magnetic  magazine  ?  How  is  a  magnetic  needle  made  ? 
What  is  said  with  regard  to  a  horse-shoe  magnet  ?  How  should  a  horse- 
shoe magnet  be  kept  ? 


240 


NATURAL  PHILOSOPHY. 


ELECTRO-MAGNETISM. 

289.  Electro-Magnetism  relates  to  magnetism  which 
is  induced  by  the  agency  of  electricity.* 

*  The  passage  of  the  two  kinds  of  electricity  (namely,  the  positive  and 
the  negative)  through  their  circuit,  is  called  the  electric  currents ;  and  the 
science  of  Electro-Magnetism  explains  the  phenomena  attending  those 
currents.  It  has  already  been  stated,  that,  from  the  connecting  wires  of 
the  galvanic  circle,  or  battery,  there  is  a  constant  current  of  electricity 
passing  from  the  zinc  to  the  copper,  and  from  the  copper  to  the  zinc  plates. 
In  the  single  circle  these  currents  will  be  negative  from  the  zinc,  and  pos- 
itive from  the  copper :  but  in  the  compound  circles,  or  the  battery,  the 
current  of  positive  electricity  will  flow  from  the  zinc  to  the  copper,  and 
the  current  of  negative  electricity  from  the  copper  to  the  zinc.  From  the 
effect  produced  by  electricity  on  the  magnetic  needle,  it  had  been  conjec- 
tured, by  a  number  of  eminent  philosophers,  that  magnetism,  or  magnetic 
attraction  is  in  some  manner  caused  by  electricity.  In  the  year  1819, 
Professor  CErsted  of  Copenhagen,  made  the  grand  discovery  of  the  power 
of  the  electric  current  to  induce  magnetism  ;  thus  proving  the  connexion 
between  magnetism  and  electricity.  In  a  short  time  after  the  discovery 
of  Professor  CErsted,  Mr.  Faraday  discovered  that  an  electrical  spark  could 
be  taken  from  a  magnet :  and  thus  the  common  source  of  magnetism  and 
electricity  was  fully  proved.  In  a  paper  published  a  few  years  ago,  this 
distinguished  philosopher  has  very  ably  maintained  the  identity  of  com- 
mon electricity,  voltaic  electricity,  magnetic  electricity,  (or  electro-mag- 
uetism,)  thermo-electricity,  and  animal  electricity.  The  phenomena  ex- 
hibited in  all  these  five  kinds  of  electricity  differ  merely  in  degree  and  the 
state  of  intensity  in  the  action  of  the  fluid.  The  discovery  of  Professor 
CErsted  has  been  followed  out  by  Ampere,  who,  by  his  mathematical  and 
experimental  researches,  has  presented  a  theory  of  the  science  less  ob- 
noxious to  objections  than  that  proposed  by  the  Professor.  The  discovery 
of  CErsted  was  limited  to  the  action  of  the  electric  current  on  needles  pre- 
viously magnetized ;  it  was  afterwards  ascertained  by  Sir  Humphrey 
Davy,  and  M.  Arago,  that  magnetism  may  be  developed  in  steel  not  pre- 


289.  Of  what  does  Electro-Magnetism  treat  ?  Note.  What  is  the  elec- 
tric current?  What  does  the  science  of  electro-magnetism  explain?  What 
is  the  difference  between  the  currents  in  the  single  and  the  compound  cir- 
cles? What  is  it  thought  causes  magnetic  attraction?  What  discovery 
W£is  made  in  the  year  1819  ?  By  whom?  What  further  discovery  was 
made  soon  after,  and  by  whom?  What  does  this  philosopher  maintain? 
How  many  kinds  of  electricity  are  there  ?  How  do  the  phenomena  ex- 
hibited in  these  five  kinds  of  electricity  differ  ? 


ELECTUO-MACNETlrfM. 


241 


290.  The  principal  facts  in  connexion  with  the  science 
•  of  electro-magnetism  are, — 

1.  That  the  electrical  current,  passing  uninierruptedhf' 
through  a  wire,  cuiniecting  the  two  ends  of  a  galvanic  hat- 
tery,  produces  an  effect  upon  the  magnetic  needle. 

2.  That  electricity  will  induce  magnetism. 

3.  That  a  magnet,  or  a  magnetic  magazine,  will  induce 
electricity.f 

4.  That  the  combined  action  of  electricity  and  magnetism, 
as  described  in  this  science,  produces  a  rotatory  motion  of 
certain  kinds  of  bodies,  in  a  direction  pointed  out  by  certain 
laws. 

5.  That  the  periodical  variation  of  the  magnetic  needle 
from  the  true  meridian,  or,  in  other  words,  the  variation  of 
the  compass,  is  caused  by  the  influence  of  the  electric  cur- 
rents. 

6.  That  the  magnetic  influence  is  not  confined  to  iron,  steel, 
&c.,  but  that  most  metals,  and  many  other  substances,  may 
be  converted  into  temporary  magnets  by  electrical  action. 

7.  That  the  magnetic  attraction  of  iron,  steel,  (fee.  may  be 
prodigiously  increased  by  electrical  agency. 

viously  possessing  it,  if  the  steel  be  placed  in  the  electric  current.  Both 
of  these  philosophers,  independently  of  each  other,  ascertained  that  the 
uniting  wire,  becoming  a  magnet,  attracts  iron  filings,  and  collects  suffi- 
cient to  acquire  the  diameter  of  a  common  quill ;  but  the  momenit  the 
connexion  is  broken,  ail  the  filings  drop  off,  and  the  attraction  diminishes 
with  the  decaying  energy  of  the  pile.  Filings  of  brass  or  copper,  or  wood 
shavings,  are  not  attracted  at  all. 

*  All  the  effects  of  electricity  and  galvanism  that  have  hitherto  been 
described,  have  been  produced  on  bodies  interposed  between  the  extremi- 
ties of  conductors,  proceeding  from  the  positive  and  negative  poles.  It 
was  not  known,  until  the  discoveries  of  Professor  CErsted  were  made,  that 
any  effect  could  be  produced  when  the  electric  circuit  is  uninterrupted. 

t  The  consideration  of  the  subject  of  electricity,  induced  by  magnetism, 
properly  belongs  to  the  subject  of  Magneto-Electricity,  in  which  connexion 
it  will  be  particularly  noticed. 

Note.  Can  magnetism  be  developed  in  steel  not  previously  possessing  it? 
Where  must  the  steel  ba  placed  ?    What  property  has  the  uniting  wire  ? 

290.  What  are  the  principal  facts  in  connexion  with  the  science  of  elec- 
tro-magnetism ?  What  is  the  first  ?  What  is  the  second  ?  What  is  the 
third?  What  is  the  fourth?  What  is  the  fifth?  What  is  the  sixth  ? 
What  is  the  seventh  ? 

11 


242 


NATURAL  PHILOSOPHY. 


8.  That  the  direction  of  the  electric  curi'ent  may,  in  all 
cases,  be  ascertained.* 

9.  That  magnetism  is  produced  whenever  concentrated  elec- 
tricity is  passed  through  space. 

lO"!  That  while  m  common  electrical  and  magnetic  attrac- 
tions and  repulsions,  those  of  the  same  name  are  mutually 
repulsive,  and  those  of  different  names  attract  each  other;  in 
the  attractions  and  repulsions  of  electric  currents,  it  is  precisely 
the  reverse,  the  repulsion  taking  place  only  when  the  wires 
are  so  situated  that  the  currents  are  in  opposite  direction. 

291.  A  magnet  freely  suspended  tends  to  assume  a 
position  at  right  angles'^  to  the  direction  of  a  current  of 
electricity  passing  near  it. 

1.  If  a  wh-e,  which  connects  the  extremities  of  a  voltaic 
battery,  be  brought  over,  and  parallel  vdih  a  magnetic  needle 
at  rest,  or  with  Its  poles  properly  directed  north  and  south, 
that  end  of  the  needle  next  to  the  negative  pole  of  the  bat- 
tery will  move  towards  the  west,  whether  the  vrire  be  on  one 
side  of  the  needle  or  the  other,  provided  only,  that  it  be 
parallel  with  it. 

Agam,  If  the  connecting  wire  be  lowered  on  either  side  of 
the  needle,  so  as  to  be  in  the  horizontal  plane  in  which  the 
needle  should  move,  it  will  not  move  in  that  plane,  but  will 
have  a  tendency  to  revolve  m  a  vertical  dhection ;  m  which, 
however,  it  will  be  prevented  from  moving,  m  consequence  of 
the  attraction  of  the  earth,  and  the  manner  in  which  it  is  sus- 
pended.   When  the  wire  is  to  the  east  of  the  needle,  the  pole 

*  This  is  done  by  means  of  the  magnetic  needle.  If  a  sheet  of  paper 
be  placed  over  a  horse-shoe  magnet,  and  fine  black  sand,  or  steel  filings, 
be  dropped  loosely  on  the  paper,  the  particles  will  be  disposed  to  arrange 
themselves  in  a  regular  order,  and  in  the  direction  of  curve  lines.  This 
is,  undoubtedly,  the  effect  of  some  infiuence,  whether  that  of  electricity, 
or  of  magnetism  alone,  cannot  at  present  be  determined. 

Where  have  the  bodies  been  supposed  to  be  placed,  in  all  the  efiects  of 
electricity  and  galvanism  that  have  hitherto  been  described?  What  is  the 
eighth  fact  in  connexion  with  the  science  of  electro-magnetism?  What  is 
the  ninth?  What  is  the  tenth?  How  can  the  direction  of  the  electric 
current  be  ascertained  ? 

291.  If  a  magnet  be  freely  suspended,  and  a  current  of  electricity  be 
passed  near  it,  what  direction  will  it  assume  1  What  illustration  of  this  is 
given  ?    What  second  illustration  is  given  ? 


ELECTRO-MAGNETISM. 


243 


nearest  to  tlie  negative  extremity  of  the  battery  will  be  ele- 
vated ;  and  when  it  is  on  the  west  side,  that  pole  will  be 
depressed. 

2.  If  the  connecting  wire  be  placed  below  the  plane  in 
which  the  needle  moves,  and  parallel  with  it,  the  pole  of  the 
needle  next  to  the  negative  end  of  the  wire  will  move  towards 
the  east ;  and  the  attractions  and  repulsions  will  be  the  re- 
verse of  those  observed  in  the  former  case."^ 

292.  The  two  sides  of  an  unmagnetized  steel  needle 
will  become  endued  with  the  north  and  south  polarity, 
if  the  needle  be  placed  parallel  with  the  connecting 
wire  of  a  voltaic  battery,  and  nearly  or  quite  in  contact 
with  it.  But  if  the  needle  be  placed  at  right  angles 
with  the  connecting  wire,  it  will  become  permanently 
magnetic ;  one  of  its  extremities  pointing  to  the  north 
pole  and  the  other  to  the  south,  when  it  is  freely  sus- 
pended and  suffered  to  vibrate  undisturbed, 

293.  Magnetism  may  be  communicated  to  iron  and 
steel  by  means  of  electricity  from  an  electrical  machine  ; 
but  the  effect  can  be  more  conveniently  produced  by 
means  of  the  voltaic  battery.  This  phenomenon  is 
called  electro-magnetic  induction. 

*  The  action  of  the  conducting-wire  in  these  cases  exhibits  a  remarka- 
ble pecuUarity.  All  other  known  forces  exerted  between  two  points,  act 
in  the  direction  of  a  straight  hne  connecting  these  points ;  and  such  is  the 
case  with  electric  and  magnetic  actions,  separately  considered  ;  but  the 
electric  current  exerts  its  magnetic  influence  laterally,  at  right  angles  to 
its  own  course.  Nor  does  the  magnetic  pole  move  either  directly  towards 
or  directly  from  the  conducting-wire,  but  tends  to  revolve  around  it  without 
changing  its  distance.  Hence  the  force  must  be  considered  as  acting  in 
the  direction  of  a  tangent  to  the  circle  in  which  the  magnetic  pole  would 
move . 


In  what  direction  will  the  pole  of  the  needle  next  to  the  negative  end 
of  the  wire  move,  if  the  connecting  wire  be  placed  below  the  plane  in 
which  the  needle  moves,  and  parallel  with  it  ?  What  is  said  with  regard 
to  the  attractions  and  repulsions  ? 

292.  How  may  the  two  sides  of  an  unmagnetized  steel  needle  become 
endued  with  the  north  and  south  polarity  ?  Under  what  circumstances  will 
It  become  permanently  magnetic  ? 

293.  How  can  magnetism  be  communicated  to  iron  and  steel?  How 
can  the  effect  be  more  conveniently  produced'? 


244 


NATURAL  rHILOSOPHY. 


1.  If  a  lielix^  be  formed  of  wire,  and  a  bar  of  steel  be  en- 
closed within  the  helix,  the  bar  will  immediately  become  mag- 
netic bv  applying  the  conducting  wires  of  the  battery  to  the 
extremities  of  the  helix.  The  electricity  from  the  common 
electrical  machine,  when  passed  through  the  helix,  will  pro- 
duce the  same  effect. 

2.  If  such  a  hehx  be  so  placed  that  it  may  move  freely,  as 
when  made  to  float  on  a  basin  of  water,  it  will  be  attracted 
and  repelled  by  the  opposite  poles  of  a  common  magnet. 

294.  If  a  magnetic  needle  be  surrounded  by  coiled 
wire,  covered  wkh  silk,  a  very  minute  portion  of  elec- 
tricity through  the  wire  will  cause  the  needle  to  deviate 
from  its  proper  direction. 

A  needle  thus  prepared,  is  called  an  electro-magnetic  mul- 
tiplier. It  is,  in  fact,  a  verv  delicate  electroscope,  or  rather 
galvanorlleter—c^A^^'MQ  of  pointing  out  the  direction  of  the 
electric  current  in  all  cases. 

295.  Among  the  most  remarkable  of  the  facts  con- 
nected with  the  science  of  electro-magnetism,  is  what 
is  called  the  electro-magnetic  rotation.  Any  wire, 
through  vrhich  a  current  of  electricity  is  passing,  has  a 
tendency  to  revolve  around  a  magnetic  pole,  in  a  plane 
perpendicular  to  the  current  ;  and  that  without  reference 
to  the  axis  of  the  magnet,  the  pole  of  which  is  used.  In 
like  manner  a  magnetic  pole  has  a  tendency  to  revolve 
around  such  a  wire. 

1.  Suppose  the  whe  perpendicular,  its  upper  end  poshive, 
or  attached  to  the  posiiive  pole  of  the  vohaic  battery,  and  its 

*  The  helix  is  a  spiral  line,  or  a  line  in  the  form  of  a  corkscrew.  The 
wire  which  forms  the  helix  should  be  coated  with  some  non-conducting 
substance,  such  as  silk  wound  around  it ;  as  it  may  then  be  formed  into 
close  coils,  without  suffering  the  electric  fluids  to  pass  from  surface  to  sur- 
face, which  would  impair  its  effect. 

What  illustration  of  this  is  given?  Note.  What  is  the  helix?  Why 
should  the  wire,  which  forms  the  helix,  be  coated  with  some  non-conduct- 
ing substance  ?  Wliat  is  said  of  a  helix,  if  it  be  placed  so  that  it  may 
move  freely? 

294.  How  can  the  magnetic  needle  be  made  to  deviate  from  its  proper 
direction  ?    What  is  a  needle  thus  prepared  called  I 

295.  What  is  the  electro-magnetic  rotation  ?    What  illustration  is  given  ? 


ELECTRO-MAGNETISM. 


245 


lower  end  nei^ativc  ;  and  let  tlio  centre  of  a  watch-dial  repre- 
sent the  niag-netic  pole  :  if  it  be  a  north  pole,  the  wire  will 
rotate  round  it  in  the  direction  that  the  hands  move;  if  it  be 
a  south  pole,  the  motion  Avill  be  in  the  opposite  direction. 
From  these  two,  the  motions  Avhich  would  take  place  if  the 
wire  were  inverted,  or  the  pole  changed,  or  made  to  move, 
may  be  readily  ascertained,  since  the  relation  now  pointed  out 
remains  constant. 

2.  Fig.  145  represents  the  ingenious  apparatus,  invented  by 
Mr.  Faraday,  to  illustrate  the  electro-magnetic  rotation.  The 
central  pillar  supports 

a  piece  of  thick  cop-  ^i^-  i^^- 

per  wh*e,  which,  on  ^ 
the  one  side,  dips  into 
the  mercury  contained 
in  a  small  glass  cup 
a.  To  a  pin  at  Ihe 
bottom  of  this  cup,  a 
small  cylindrical  mag- 
net is  attached  by  a 
piece  of  thread,  so  that 
one  pole  shall  rise  a 
little  above  the  sur- 
face of  the  mercury, 
and  be  at  liberty  to 

move  around  the  wire.  The  bottom  of  the  cup  is  perforated, 
and  has  a  copper  pin  passing  through  it,  which,  touching  the 
mercury  on  the  inside,  is  also  in  contact  with  the  wire  that 
proceeds  outwards,  on  that  side  of  the  instrument.  On  the 
other  side  of  the  instrument^  h,  the  thick  copper  wire,  soon 
after  turning  down,  terminates,  but  a  thinner  piece  of  wire 
forms  a  communication  between  it  and  the  mercury  on  the 
cup  beneath.  As  freedom  of  motion  is  regarded  in  the  wire, 
it  is  made  to  communicate  with  the  former  by  a  ball  and 
socket-joint,  the  ball  being  held  in  the  socket  by  a  piece  of 
thread ;  or  else  the  ends  are  bent  into  hooks,  and  the  one  is 
then  hooked  to  the  other.  As  good  metallic  contact  is  re- 
quired, the  paj'ts  should  be  amalgamated,  and  a  small  di  op 
of  mercury  placed  between  them ;  the  low^er  ends  of  the  wire 


What  does  Fig.  145  represent?  Explain  the  figure.  How  is  the  free- 
dom of  motion,  which  is  required  on  the  wire,  obtained?  How  can  the 
metallic  contact  which  is  required  be  obtained? 


246 


NATL^RAL  PHILOSOPHY. 


should  also  be  amalgamated.  Beneath  the  hanging  wire,  a 
small  circular  magnet  is  fixed  in  the  socket  of  the  cup  b,  so 
that  one  of  its  poles  is  a  little  above  the  mercury.  As  in  the 
former  cup,  a  metallic  connexion  is  made  through  the  bottom, 
from  the  mercui  y  to  the  external  wire. 

If  now  the  poles  of  a  battery  be  connected  with  the  hori- 
zontal external  wires,  cc,  the  current  of  electricity  will  be 
through  the  mercury  and  the  horizontal  wire,  on  the  pillar 
which  connects  them,  and  it  Avill  now  be  found,  that  the 
moveable  part  of  the  wire  will  rotate  around  the  magnetic 
pole  in  the  cup  b,  and  the  magnetic  pole  round  the  fixed  wire 
in  the  other  cup  a,  in  the  direction  before  mentioned. 

By  using  a  very  dehcate  apparatus,  the  magnetic  pole  of 
the  earth  may  be  made  to  put  the  wire  in  motion. 

3.  Fig.  146  represents  another  ingeniotis 
contrivance,  invented  by  M.  Ampei-e,  for  illus- 
trating the  electro-magnetic  rotation ;  and  it 
has  the  advantage  of  comprising  within  itself 
the  voltaic  combination  which  is  employed.  It 
consists  of  a  cyhnder  of  copper  about  two  inches 
high,  and  a  liule  less  than  two  inches  internal 
diameter,  within  which  is  a  small  cylinder,  about 
one  inch  in  diameter.  The  two  cylinders  are  con- 
nected together  by  a  bottom,  ha^^ng  an  aperture 
in  its  centre  the  size  of  the  smaller  cylinder, 
leaving  a  circular  cell,  which  may  be  filled  with 
acid.  A  piece  of  strong  copper  wire  is  fastened 
aci  oss  the  top  of  the  inner  cylinder,  and  from  the  middle  of 
it  rises,  at  a  right  angle,  a  piece  of  copper  wire,  supporting  a 
very  small  metal  cup,  containing  a  few  globules  of  mercury. 
A  cylinder  of  zinc,  open  at  each  end,  and  about  an  inch  and 
a  qiiarter  in  diameter,  completes  the  voltaic  combination.  To 
the  latter  cylinder  a  wire,  bent  like  an  inverted  U,  is  soldered 
at  opposite  sides;  and  in  the  bend  of  this  wire  a  metallic 
point  is  fixed,  which,  when  inserted  in  the  little  cup  of  mer- 
cury, suspends  the  zinc  cylinder  in  the  cell,  and  allows  it  a 
free  circular  motion.    An  additional  point  is  directed  doAvn- 


If  the  poles  of  a  battery  be  coiiiiected  with  the  horizontal  external  wires, 
c  c,  throughout,  what  direction  will  the  current  of  electricity  take  ?  Round 
what  pole  will  the  moveable  part  of  the  wire  rotate  ?  Round  what  will 
the  magnetic  pole  rotate  ?  What  does  Fig.  146  represent  ?  Of  what  does 
it  consist? 


ELECTRO-MAGNETISM. 


247 


wards  from  the  central  part  of  the  stronger  wire,  whicli  point 
is  adapted  to  a  small  hole  at  the  top  of  a  powerful  bar  mag- 
net. When  the  apparatus  with  one  point  only  is  charged 
with  diluted  acid,  and  set  on  the  magnet  placed  vertically,  the 
zinc  cylinde]'  revolves  in  a  direction  determined  by  the  mag- 
netic pole  which  is  uppermost.  With  two  points,  the  copper 
revolves  in  one  direction,  and  the  zinc  in  a  contrai  y  direction. 

4.  If,  instead  of  a  bar  magnet,  a  horse-shoe  magnet  be  em- 
ployed, with  an  apparatus  on  each  pole  similar  to  that  which 
has  now  been  described,  the  cylinders  in  each  will  revolve  ia 
opposite  directions.  The  small  cups  of  mercury  mentioned  in 
the  preceding  description  are  sometimes  omitted,  and  the 
points  are  inserted  in  an  indentation  on  the  inverted  U."^ 

296.  The  magnetizing  power  of  the  conducting  wires 
of  a  battery  is  very  greatly  increased  by  coiling  it  into 
a  helix,  into  which  the  body  to  be  magnetized  may  be 
inserted.  A  single  circular  turn  is  more  efficient  than  a 
straight  wire,  and  each  turn  adds  to  the  power  within  a 
certain  lin:iit,  whether  the  whole  forms  a  single  layer,  or 
w^hether  each  successive  turn  encloses  the  previous  one. 

1.  When  a  helix  of  great  power  is  required,  it  is  composed 
of  several  layers  of  wire.  The  wire  forming  the  coil  must  be 
insulated  by  being  wound  with  cotton,  to  prevent  any  lateral 
passage  of  the  current. 

2.  Fig\  147  represents  a  helix  on  a  stand.  A  bar  of  soft 
iron,  N  S,  being  placed  within  the  helix,  is  connected  w^ith  the 

*  The  phenomenon  of  electro-magnetic  rotation  is  beautifully  illustrated 
by  Mr.  Davis,  in  his  treatise  on  Magnetism,  to  which  reference  has  al- 
ready been  made.  He  has  invented  and  prepared  a  great  variety  of  in- 
genious contrivances  for  the  illustration  of  this  subject,  and  his  book  should 
be  in  the  hands  of  all  who  desire  a  thorough  acquaintance  with  all  that 
has  been  discovered  in  the  new  department  of  science  in  which  magnetism 
and  electricity  are  combined.  The  author  has  been  indebted  to  Mr.  Davis' 
volume  for  a  number  of  explanations  which  are  incorporated  in  this  work. 


How  will  the  cylinders  in  each  revolve,  if,  instead  of  a  bar-magnet, 
a  horse-shoe  magnet  be  employed,  with  an  apparatus  on  each  pole  shnilar 
to  that  which  has  now  been  described? 

296.  How  may  the  magnetizing  power  of  the  connecting  wires  be  in- 
creased? Is  a  single  circuit  preferable  to  a  straight  wire?  Explain  Fig. 
147. 


248 


NATURAL  PHILOSOPHY. 


battery  by  means  of  the  screw-cups  on  the  base  of  the  stand. 
The  two  extremities  of  the  bar  instantly  become  strongly 

Fig.  147. 


magnetic,  and  keys,  or  pieces  of  iron,  iron  filings,  nails,  <^c. 
will  be  held  up  so  long  as  the  connexion  with  the  ba.ttery  is 
sustained.  But  so  soon  as  the  connexion  is  broken,  the  bar 
loses  its  magnetic  power,  and  the  suspended  articles  will  fall. 
The  bar  can  be  made  alternately  to  take  up  and  drop  such 
magnetizable  articles  as  are  brought  near  it,  as  the  connexion 
with  the  battery  is  made  or  broken. 

3.  A  steel  bar  placed  within  the  hehx  acquires  the  polarity 
less  readily,  btit  retains  it  after  the  connexion  is  broken. 
Small  rods  or  bars  of  steel,  needles,  dzc.  may  be  made  perma- 
nent magnets  in  this  way. 

297.  A  bar  temporarily  magnetized  by  the  electric 
current  is  called  an  electro-magnet. 

To  ascertain  the  poles  of  an  electro-magnet  it  must  be  ob- 
served that  the  north  jjole  will  he  at  the  farthest  end  of  the  helix 
when  the  current  circulates  in  the  direction  of  tJie  hands  of  a 
watch. 

298.  Magnets  of  prodigious  power  have  been  formed 
by  means  of  voltaic  electricity. 

297.  What  is  a  bar  called  that  is  temporarily  magnetized  ?  How  do  you 
ascertain  the  poles  of  an  electro-magnet  ? 

298.  How  have  magnets  of  great  power  been  formed? 


ELECTRO-MAGNETISM. 


249 


1.  An  electro-magnet  was  constructed  by  Professor  Henry 
and  Dr.  Ten  Eyck,  whicli  was  capable  of  supporting  a  weight 
of  750  pounds.  They  have  subsequently  constructed  another, 
wliich  will  sustain  2063  pounds.  It  consists  of  a  bar  of  soft 
iron,  bent  into  the  form  of  a  horse-shoe,  and  wound  with 
twenty-six  strands  of  copper  bell-wire,  covered  with  cotton 
thi-eads,  each  thirty-one  feet  long;  about  eighteen  inches  of 
the  ends  are  left  projecting,  so  that  only  twenty-eight  feet  of 
each  actually  surround  the  iron.  The  aggregate  length  of  the 
coils  is  therefore  728  feet.  Each  strand  is  wound  on  a  httle  less 
than  an  inch :  in  the  middle  of  the  horse-shoe  it  forms  three 
thicknesses  of  wii  e ;  and  on  the  ends,  or  near  the  poles,  it  is 
wound  so  as  to  form  six  thicknesses.  Being  connected  with  a 
battery  consisting  of  plates,  containing  a  little  less  than  forty- 
eight  square  feet  of  surface,  the  magnet  supported  the  prodi- 
gious weight  stated  above,  namely,  2063  pounds. 

2.  Heliacal  Ring. 

Fig.  148  represents  a  Fig.  148. 

heliacal  ring,  or  ring  of 
wiie  bent  in  the  form  of 
a  hehx,  with  the  ends 
of  the  wire  left  free  to 
be  inserted  in  the  screw- 
cups  of  a  battery.  Two 
semicircular  pieces  of 
soft  unmagnetized  iron, 
furnished  with  rings — 
the  upper  one  for  the 
hand,  the  lower  one  for 
weights — are  prepared 
to  be  inserted  info  the 
helix,  in  the  manner  of 
the  links  of  a  chain.  As 
soon  as  the  ends  of  the 
helix  are  inserted  into 
the  screw-cups  of  the 
battery,  the  rings  will 
be  held  together  with 
great  force,  by  magnetic  attraction.^ 

*  That  the  attraction  is  caused,  or  that  the  magnetism  is  induced  by 


What  weight  was  the  magnet  constructed  by  Professor  Henry  and  Dr 
Ten  Eyck  capable  of  supporting?  Explain  the  hehacal  ring  and  Fig.  148. 


250 


NATURAL  PHILOSOPHY 


COMMUNICATION  OF  MAGNETISM   TO  STEEL  BY  THE 
ELECTRO-MAGNET. 

299.  Bars  of  the  U  form  are  most  readily  magnetized 
by  drawing  them  from  the  bend  to  the  extremities, 
across  the  poles  of  the  U  electro-magnet,  in  such  a 
way  that  both  halves  of  the  bar  may  pass  at  the  same 
time  over  the  poles  to  which  they  are  applied.  This 
should  be  repeated  several  times,  recollecting  always  to 
draw  the  bar  in  the  same  direction. 

Fig.  149  represents  the  U  electro-magnet,  witli  the  bar  to 
be  magnetized.    When  the  bar  is  thick  belli  surfaces  should 


Fig.  149 


be  drawn  across  the  electro-magnet,  keeping  e-:cli  half  applied 
to  the  same  pole.  To  remove  the  magnetism  it  is  only  neces- 
sary to  reverse  the  process  by  which  it  was  magnetized,  that 
is,  to  draw  the  bar  across  the  electro -magnet  in  a  contrary 
direction. 

the  circulation  of  electricity  around  the  coils,  may  be  proved  by  the  fol- 
lowing interesting  experiment.  Hold  the  heliacal  ring  horizontally  over 
a  plate  of  small  nails,  and  suspend  an  unmagnetizcd  bar  perpendicu- 
larly on  the  outside  of  the  ring,  over  the  nails,  and  there  will  be  no  at- 
traction. Suspend  the  bar  perpendicularly  through  the  helix,  and  the 
nails  will  all  attach  themselves  to  it  in  the  form  of  tangents  to  the  circles 
formed  by  the  coils  of  the  heliacal  ring. 


299  How  are  bars  of  the  U  form  most  readily  magnetized?  Explain 
Fig.  149. 


ELECTRO-MAGNETISM. 


251 


THE  ELECTRO-MAGNETIC  TELEGRAPH. 

300.  From  the  description  which  has  now  been  given 
of  the  electro-magnetic  power,  it  will  readily  be  perceived 
that  a  great  force  can  be  made  to  act,  simply  by  bring- 
ing a  wire  into  contact  with  another  conductor,  and 
that  the  force  can  be  instantly  arrested  in  its  operation 
by  removing  the  wire  from  the  contact ;  in  other  words, 
that  by  connecting  and  disconnecting  a  helix  with  a 
battery  a  prodigious  power  can  be  made  successively 
to  act  and  cease  to  act.  Advantage  has  been  taken  of 
this  principle  in  the  construction  of  the  AmiCrican  elec- 
tro-magnetic telegraph,  which  was  matured  by  Professor 
Morse,  and  first  put  into  operation  between  the  cities  of 
Baltimore  and  Washington,  in  1844.^  It  comes  not 
within  the  province  of  this  work  to  enter  into  a  minute 
description  of  this  great  invention,  but  the  principles  of 
its  construction  may  be  briefly  stated  as  follows : 

1.  An  electro-magnet  is  so  arranged  with  its  armature,  that 
when  the  armature  is  attracted  it  communicates  its  motion  to 
a  lever,  to  which  a  blunt  point  is  attached,  which  marks  a 
narrow  .strip  of  paper,  drawn  under  it  by  machinery  resem- 
bling clock-work,  whenever  the  electro-magnet  is  in  action. 
When  the  electro -miagnet  ceases  to  act,  the  armature  falls, 
and  communicating  its  motion  to  the  lever,  the  blunt  point  is 
removed  from  its  contact  with  the  paper.  By  this  means,  if 
one  of  the  wires  from  the  battery  is  attached  to  the  screw- 
cup,  whenever  the  other  wire  is  attached  to  the  remaining  cup, 
the  armature  is  powerfully  attracted  by  the  magnet,  and  the 

*  For  a  particular  description  of  this  wonderful  invention,  the  student 
is  referred  to  Davis'  treatise  on  Magnetism,  in  which  the  parts  are  all  de- 
scribed with  a  minuteness  which  leaves  little  more  to  be  desired.  The 
history,  also,  of  the  successive  steps  by  which  it  was  brought  to  its  present 
degree  of  perfection,  is  also  to  be  found  in  the  same  connexion.  It  will 
be  sufficient  here  to  state,  that  it  was  not  until  Professor  Hcuiry  of  Prince- 
ton, N.  J.,  had  discovered  the  mode  of  constructing  the  powerful  electro- 
magnets which  have  been  described,  that  this  form  of  telegraph  became 
possible. 


300.  Explain  the  electro-magnetic  telegraph. 


252  NATURAL  PHILO>SOPHY. 

point  on  the  lever  presses  the  paper  into  the  groove  of  a 
roller,  thereby  making  an  indentation  on  the  paper  corre- 
sponding in  length  to  the  time  during  which  the  contact  with 
the  battery  is  maintained,  the  paper  being  drawn  slowly  un- 
der the  roller. 

An  alphabet  of  signs  or  symbols,  is  formed  by  indentations 
of  the  paper  varying  in  length,  which  is  easily  read  by  those 
connected  with  the  telegraph.  Thus,  the  letter  e  is  repre- 
sented by  one  short  mark  thus  - ;  the  letter  o,  by  two  marks 

thus  -  - ;  the  letter  a,  by  a  short  and  a  long  one  thus  ; 

the  letter  /  by  a  short,  a  long,  and  a  short  one,  thus  ; 

the  figui-e  1  by  a  short,  two  long,  and  a  short  one,  thus 

 •    By  such  an  arrangement  all  the  letters  and  the 

numerals  are  represented  by  the  telegraph.^  A  simple  con- 
trivance connected  with  the  machinery  causes  a  bell  to  strike 
when  the  telegraph  commences  its  operations,  and  thus  gives 
warning  to  the  attendant. 

2.  It  has  already  been  stated  that  the  motion  of  electricity 
exceeds  in  velocity  even  that  of  light,  and  that  its  velocity  is 
equal  to  288,000  miles  in  a  second  of  time.  The  invention 
that  thus  enables  man  to  communicate  with  his  brother  man, 
with  a  rapidity  that  sets  time  and  space  at  defiance,  de- 
servedly ranks  as  one  of  the  greatest  ever  achieved  by  human 
ingenuity.j- 

*  The  following  table  presents  a  view  of  Morse's  Telegraphic  Alphabet 


A  -—  B   C  --   -  D   E  - 

F  .    G   H  I  J  

K   L    M   N  — -  O  -  - 

P   Q   R  -  S  --.  T  - 

U   V   W   X   Y--- 

Z  -  -  -   -  &  -   .  - .  &c.  

Numerals. 

1   2   3   4   5  

6   7   8   9   0   


t  The  invention  of  the  magnetic  telegraph  has  been  made  since  the 
first  edition  of  this  work  was  published.  The  subject  of  Electro-Magnet- 
ism in  the  earliest  editions,  was  closed  by  the  following  remarks.  So 
early  a  fulfilment  of  any  portion  of  them  is  remarkable  and  unexpected. 

The  science  of  Electro-Magnetism  is  yet  in  its  infancy.  The  discoveries 
which  have  rewarded  the  labors  of  philosophical  research  are  truly  won- 
derful ;  hut  man  has  as  yet  hut  lifted  the  veil,  hehind  which  the  stu- 
pendous operations  of  nature  are  carried  on.  What  wonders  he  will 
discover,  should  he  penetrate  the  recesses  of  her  laboratory,  imagination 
cannot  conceive.    It  would  have  excited  no  little  surprise  among  the 


MAGNETU-ELECTKICITY. 


253 


3.^  Electro-Magnetism  as  a  Motive  Power. — The  first 
application  of  elcctro-in.i^iici istn  ;is  u  motive  power  was  made 
by  Professor  Henry,  but  he  made  no  attempt  to  apply  it  to 
practical  purposes.  Dr.  Page  has  gone  fuither,  and  sug- 
gested improvements  which  iiave  ovei-conie  some  of  the  ob- 
stacles to  the  employment  of  this  power.  But  thus  far  it  has 
been  found  that  the  smallest  machines  present  the  strongest 
probabihty  of  this  power  becoming  available. 

MAGNETO-ELECTRICITY. 

301.  Magneto-Electricity  treats  of  the  development  of 
electricity  by  magnetism. 

Electric  currents  are  excited  in  a  conductor  of  elec- 
tricity by  magnetic  changes  taking  place  in  its  vicinity. 
Thus,  the  movement  of  a  magnet  near  a  metallic  wire, 
or  near  an  iron  bar  enclosed  in  a  wire  coil,  occasions 
currents  in  the  wire. 

1.  When  an  armature,  or  any  piece  of  soft  iron,  is  brought 
into  contact  with  one  or  both  of  the  poles  of  a  magnet,  it 
becomes  itself  magnetic  by  induction,  and  by  its  reaction  adds 
to  the  power  of  the  magnet :  on  the  contrary,  when  removed 
from  the  contact,  it  diminishes  the  power  of  the  magnet,  and 
these  alternate  changes  in  its  magnetic  state  induce  a  current 
of  electricity. 

2.  The  most  powerful  effects  are  obtained  by  causing  a  bar 
of  soft  iron,  enclosed  in  a  helix,  to  revolve  hy  mechanical  means 
near  the  poles  of  a  steel  magnet.  As  the  iron  approaches 
the  poles,  in  its  revolution,  it  becomes  magnetic ;  as  it  recedes 

philosophers  of  the  last  century,  had  the  opinion  been  advanced,  that  elec- 
tricity and  magnetism  are  identical.  Perhaps  the  future  philosopher  may 
surprise  a  generation  not  very  distant,  by  the  annunciation  of  the  discov- 
ery, that  attraction  and  repulsion  of  all  kinds  are  to  be  traced  to  a  common 
source  ;  that  the  same  influence  which  binds  the  particles  of  a  grain  of 
sand  together,  is  seen  in  the  vivid  flash  which  causes  the  *  lit  lake  to 
shine  ;'  or  heard  in  the  '  live  thunder,'  as  it  leaps  from  peak  to  peak  ;  or 
known  in  the  unerring  guide  which  it  furnishes  the  mariner  in  his  course 
over  the  trackless  deep  ;  and  admired  in  the  inusic  of  the  spheres,  as  they 
harmoniously  roll  in  grand  and  majestic  course  in  the  immeasurable  re- 
gions of  infinite  space." 


301.  Of  what  does  magneto-electricity  treat  ?  How  are  electric  cur- 
rents excited  ? 


254 


NATURAL  PHILOSOPHY. 


from  them,  its  magnetism  disappears :  and  this  alternation  of 
magnetic  states  causes  the  flow  of  a  cmTent  of  electricity, 
which  may  be  directed  in  its  course  to  screw-cups,  from 
which  it  may  be  received  by  means  of  wires  connected  with 
the  cups. 

3.  The  Magxeto-Electric  Machine. — Fig.  150  represents 
the  magneto-electric  machine,  in  which  an  armature,  bent 

Fig.  150. 


twice  at  right  angles,  is  made  to  revolve  rapidly  in  front  of 
the  poles  of  a  compound  steel  magnet  of  the  U  form.  The 
U  magnet,  whose  north  pole  is  seen  at  N,  is  fixed  in  a  hori- 
zontal position,  with  its  poles  as  near  the  ends  of  the  armature 
as  will  allow  the  latter  to  rotate  without  coming  into  contact 
with  them.  The  armature  is  mounted  on  an  axis,  extending 
from  the  pill:ir  P  to  a  small  pillar  between  the  poles  of  the 
magnet.  Each  of  its  legs  is  enclosed  in  a  helix  of  fine  insu- 
lated wire.  The  upper  part  of  the  pillar  P  slides  over  the 
lower  part,  and  can  be  fastened  in  any  position  by  a  binding 
screw.  In  this  way  the  band  connecting  the  tvro  wheels  may 
be  tightened  at  pleasure,  by  increasing  the  distance  between 
them.  This  arrangement  also  renders  the  machine  more  port- 
able. By  means  of  the  multiplving-wheel  W,  which  is  con- 
nected by  a  band  with  a  small  wheel  on  the  axis,  the  armature 
is  made  to  revolve  rapidly,  so  that  the  magnetism  induced  in 
it  by  the  steel  magnet  is  alternately  destroyed  and  renewed  in 


Explain  the  magueto-electric  machine,  Fig.  150. 


THERMO-ELECTRICITY. 


255 


a  reverse  direction  to  tlu^  pixn  ions  one.  When  the  legs  of 
the  armature  are  appioachitig  the  mainiet,  the  one  opposite 
the  north  pole  acquiies  south  polarity,  and  tlie  other  north 
polarity.  The  magnetic  power  is  greatest  while  the  armature 
is  passing  in  front  of  the  poles.  It  gradually  diminishes  as 
the  armature  leaves  this  position,  and  nearly  disappears  when 
it  stands  at  right  angles  with  the  magnet.  As  each  leg  of 
the  armature  approaches  the  other  pole  of  the  U  magnet,  by 
the  continuance  of  the  motion,  magnetism  is  again  induced  in 
it,  but  in  the  reverse  direction  to  the  previous  one.  These 
changes  in  the  magnetic  state  of  the  armature  excite  electric 
currents  in  the  surrounding  helices,  powerful  in  proportion 
to  the  rapidity  with  which  the  magnetic  changes  are  pro- 
duced. 

4.  Shocks  may  thus  be  obtained  from  the  machine,  and  if 
the  motion  is  very  rapid,  in  a  powerful  machine,  the  torrent 
of  shocks  becomes  insupportable — the  muscles  of  the  hands 
which  grasp  the  handles  are  involuntarily  contracted,  so  that 
it  is  impossible  to  loosen  the  hold.  The  shocks,  however,  are 
instantly  suspended  by  bringing  the  metallic  handles  into  con- 
tact* 

THERMO-ELECTRICITY. 

302.  Thermo-electricity-  expresses  a  form  of  elec- 
tricity developed  by  the  agency  of  heat.f 

*  For  a  further  explanation  of  the  magneto-electric  machine,  the  stu- 
dent is  referred  to  Davis'  excellent  "  Manual  of  Magnetism ;"  a  volume 
which  ought  to  be  in  the  hands  of  all  who  wish  to  become  acquainted  with 
the  subjects  of  magnetism  and  chemical  electricity. 

t  In  the  year  1822,  Professor  Seebeck,  of  Berlin,  discovered  that  cur- 
rents of  electricity  might  be  produced  by  the  partial  application  of  heat  to 
a  circuit  composed  exclusively  of  solid  conductors.  The  electrical  cur- 
rent, thus  excited,  has  been  termed  Thermo -electric,  (from  the  Greek 
Thermos,  which  signifies  heat,)  to  distinguish  it  from  the  common  galvanic 
current ;  which,  as  it  requires  the  intervention  of  a,  fluid  element,  was  de- 
nominated a  Hydro-electric  current.  The  term  Stereo-electric  current 
has  also  been  applied  to  the  former,  in  order  to  mark  its  being  produced  in 
systems  formed  of  solid  bodies  alone.  It  is  evident  that  if,  as  is  supposed 
in  the  theory  of  Ampere,  magnets  owe  their  peculiar  properties  to  the  con- 


302.  What  is  thermo-electricity  ? 


256 


NATURAL  PHILOSOPHY. 


1.  If  tlie  junction  of  two  dissimilar  metals  be  heated,  an 
electrical  current  will  flow  from  the  one  to  the  other. 

2.  Instead  of  two  different  metals,  one  metal  in  different 
conditions  can  be  used  to  excite  the  current. 

3.  Metals  differ  greatly  in  their  power  to  excite  a  current, 
when  associated  in  thermo-electric  pairs.  A  current  may  be 
excited  with  two  wires  of  the  same  metal,  by  heating  the  end 
of  one,  and  bringing  it  into  contact  with  the  other.  This  ex- 
periment is  most  successful  when  metals  are  used  that  have 
the  lowest  conducting  power  of  heat. 

4.  Thermo-electric  batteries  have  been  constructed,  with  suf- 
ficient power  to  give  shocks  and  sparks,  and  produce  various 
magnetic  phenomena,  indicative  of  great  magnetic  power.* 

tinual  circulation  of  electric  currents  in  their  minute  parts,  these  currents 
will  come  under  the  description  of  the  stereo-electric  currents. 

From  the  views  of  electricity  which  have  now  been  given,  it  appears 
that  there  are,  strictly  speaking,  three  states  of  electricity.  That  derived 
from  the  common  electrical  machine  is  in  the  highest  degree  of  tension, 
and  accumulates  until  it  is  able  to  force  its  way  through  the  air,  which  is 
a  perfect  non-conductor.  In  the  galvanic  apparatus,  the  currents  have  a 
smaller  degree  of  tension  ;  because,  although  they  pass  freely  through  the 
metallic  elem.ents,  they  meet  with  some  impediments  in  traversing  the 
fluid  conductor.  But  in  the  thermo-electric  currents,  the  tension  is  re- 
duced to  nothing ;  because,  throughout  the  whole  course  of  the  circuit,  no 
impediment  exists  to  its  free  and  uniform  circulation. 

*  The  subject  of  thermo-electricity  is  more  fully  treated  in  Davis'  • 
"  Manual  of  Magnetism,"  to  which  reference  has  already  been  made,  and 
to  which  the  author  acknowledges  he  has  been  indebted  for  much  inform- 
ation in  the  departments  of  Electricity  and  Magnetism. 


Note.  To  what  do  the  magnets  owe  their  peculiar  properties?  What 
follows  from  this  ?  How  many  states  of  electricity  are  there  ?  What  is 
said  of  that  derived  from  the  common  electrical  machine?  What  is  said 
of  that  derived  from  the  galvanic  apparatus?  What  is  said  of  the  thermo- 
electric  currents? 


ASTRONOMY. 


257 


CHAPTER  XIII. 

ASTRONOMY,* 

303.  Aatronomy  treats  of  the  heavenly  bodies,  such 
as  the  sun.  moon,  stars,  comets,  planets,  &c. 

The  earth  on  which  we  live  is  a  large  globe,  or  ball,  nearly 
eiglit  thousand  miles  in  diameter,  and  about  tv>^enty-five  thou- 
sand miles  in  circumference.  It  is  known  to  be  round, — first, 
because  it  casts  a  circular  shadow,  which  is  seen  on  the  moon, 
during  an  eclipse ;  secondly,  because  the  upper  parts  of  distant 
objects  on  its  surface  can  be  seen  at  the  greatest  distance ; 
thirdly,  it  has  been  circumnavigated.  It  is  situated  in  the 
midst  of  the  heavenly  bodies,  which  we  see  around  us  at 
night,  and  forms  one  of  the  number  of  those  bodies ;  and  it 
belongs  to  that  system,  which,  having  the  sun  for  its  centre, 
and  being  influenced  by  its  attraction,  is  called  the  solar\ 
system. 

304.  The  solar  system  consists  of  the  sun,  which  is  in 
the  centre  ; 

Of  eight  primary  planets,  named  Mercury,  Venus, 
Earth,  Mars,  Jupiter,  Saturn,  Uranus,  and  Neptune  ;J 

Of  four  Asteroids,  or  smaller  planets,  namely,  Ceres, 
Pallas,  Juno,  and  Vesta ; 

Of  eighteen  secondary  planets  or  moons,  of  which 

*  It  is  proper  here  to  remark,  that  many  of  the  branches  of  Natural 
Philosophy  require  in  the  student  an  intimate  acquaintance  with  the  prin- 
ciples of  mathematical  science.  This  is  particularly  the  case  with  as- 
tronomy. As  this  book  is  designed  for  those  who  have  made  little  progress 
in  the  mathematics,  the  following  treatise  on  astronomy  contains  those 
facts  and  principles  only  of  the  science  which  are  intelligible,  without  the 
aid  of  mathematical  light. 

t  The  word  solar  means  belonging  to  the  sun. 

\  This  planet  was  very  recently  discovered  by  Verner,  whose  name  at 
first  was  applied  to  it.    It  is  now  more  generally  called  Neptune. 


303.  Of  what  does  Astronomy  treat  ?  What  is  said  of  tlie  earth  ?  How 
is  the  earth  known  to  be  round?    Where  is  it  situated? 

304.  Of  what  does  the  solar  systciii  consist? 


258 


NATURAL  PHILOSOPHY. 


the  Earth  has  one,  Jupiter  four,  Saturn  seven,  and 
Uranus  six  ;  and 

Of  an  unknown  number  of  comets.* 

The  stars,  which  we  see  in  the  night  time,  are  supposed  to  be 
suns,  surrounded  by  systems  of  pkmets,  too  distant  to  be  seen 
from  the  earth.  Ahhough  they  appear  so  numerous  on  a 
bright  night,  they  become  much  more  so  by  the  aid  of  glasses. 

305.  The  planetsf  may  be  distinguished  from  the  stars 
by  their  steady  light ;  while  the  stars  appear  to  twinkle. 
The  planets,  likewise,  seem  to  change  their  relative 
places  in  the  heavens,  while  those  luminous  bodies 
which  are  called  fixed  stars  appear  to  preserve  the  same 
relative  position. 

1.  The  sun,  the  moon,  the  planets,  and  the  fixed  stars, 
which  appear  to  us  so  small,  are  supposed  to  be  large  worlds, 
of  various  sizes,  and  at  different  but  immense  distances  from 
us»  The  reason  that  they  appear  to  us  so  small  is,  that  on 
account  of  their  immense  distances  they  are  seen  under  a  small 
angle  of  vision. 

2.  It  has  been  stated  in  the  early  pages  of  this  book,  that 
every  portion  of  ^natter  is  attracted  by  every  other  portion, — 
and  that  the  force  of  the  attraction  depends  upon  the  quantity 

*  The  planet  Neptune,  so  recently  discovered,  is  undoubtedly  attended 
by  his  secondaries  or  moons,  two  of  which  have  already  been  discovered." 
Whether  more  will  ever  be  discovered,  is  a  question  which  remains  for  the 
future  astronomer  to  answer. 

t  The  meaning  of  the  word  planet  is  properly  a  wanderer^  or  a  wan- 
dering star.  These  luminaries  were  so  called  because  they  never  retain 
the  same  situation,  but  are  constantly  changing  their  relative  positions  ; 
while  those  stars  which  appear  to  retain  their  places  are  GciWed  fixed  stars. 
The  cause  of  the  motion  of  the  planets  will  be  presently  explained. 


What  are  the  stars  supposed  to  be  ?  Do  we  see  more  by  the  aid  of 
glasses  than  without? 

305.  How  may  the  planets  be  distinguished  from  the  stars?  How  are 
the  planets  distinguished  from  the  fixed  stars?  What  is  the  mean- 
ing of  the  word  planet?  Why  are  they  called  planets?  What  are  the 
fixed  stars?  What  are  the  sun,  moon,  planets,  and  fixed  stars  supposed 
to  be  ?  Why  do  they  appear  so  small  ?  What  has  been  stated  with  re- 
gard to  the  attraction  of  portions  of  matter?  Upon  what  force  does  this 
attraction  depend  ? 


ASTRONOMY.  259 

of  matter  and  tlie  distance.  As  attraction  is  mutual,  Ave  find  tliat 
all  of  the  heavenly  bodies  attract  the  earth  ;  and  the  earth,  like- 
wise, attracts  all  of  the  heavenly  bodies.  It  has  been  proved,  that 
a  body  when  actuated  by  several  forces  will  not  obey  either  one, 
but  will  move  in  a  direction  hetweeii  them.  It  is  so  with  the 
heavenly  bodies, — each  one  of  them  is  attracted  by  every 
other  one ;  and  these  attractions  ai  e  so  nicely  balanced  by 
creative  wisdom,  that,  instead  of  rushing  together  in  one  mass, 
they  are  caused  to  move  in  regular  paths,  (called  orbits,) 
around  a  central  body  ;  which  being  attracted  in  different  di- 
rections, by  the  bodies  w^iich  revolve  around  it,  will  itself  re- 
volve around  the  centie  of  gravity  of  the  system.  Thus,  the 
sun  is  the  centre  of  what  is  called  the  solar  system,  and  the 
planets  revolve  around  it  in  different  tunes,  at  different  distances, 
and  with  different  velocities. 

306.  The  paths  or  courses  in  which  the  planets  move 
around  the  sun  are  called  their  orbits. 

1.  In  obedience  to  the  universal  law  of  gravitation,  the 
planets  revolve  around  the  sun  as  the  centre  of  their  system ; 
and  the  time  that  each  one  takes  to  perform  an  entire  revolu- 
tion  is  called  its  year.  Thus,  the  planet  Mercury  revolves 
around  the  sun  in  87  of  our  days.  Hence,  a  year  on  that 
planet  is  equal  to  87  days.  The  planet  Venus  revolves  around 
the  sun  in  224  days.  That  is,  therefore,  the  length  of  the 
year  of  that  planet.  Our  earth  revolves  around  the  sun  in 
about  3G5  days  and  6  hours.  Our  year,  therefore,  is  of  that 
length. 

2.  The  length  of  time  that  each  planet  takes  in  performing- 
its  revolution  around  the  sun,  or,  in  other  words,  the  leno-rh  of 
the  year  on  each  planet,  is  as  follows.  {The  fractional  parts  of 
the  day  are  omitted.) 

What  follows  from  attraction  being  mutual?  What  direction  do  bodies 
take  v^hen  actuated  by  several  forces?  Is  this  true  with  regard  to  the 
heavenly  bodies  ?  What  is  the  centre  of  the  solar  system?  What  is  said 
of  the  revolution  of  the  planets  ? 

306.  What  are  the  paths  in  which  the  planets  move  around  the  sun 
called  ?  Around  what  do  the  planets  revolve  ?  What  is  a  year  on  each 
planet?  How  long  is  the  year  of  the  planet  Mercury?  How  long  is  the 
planet  Venus  performing  her  revolution  around  the  sun?  How  long  is 
the  earth  performing  her  revolution  around  the  sun?  What  is  the  length 
of  the  year  on  the  planet  Mercury?  Venus?  Earth?  Mars?  Vesta? 
Jun;7   Ceres?   Pallas?   Jupiter?  Saturn?   Herschel?  Neptune? 


260 


NATURAL  PHILOSOPHY, 


Days.  Days.  Days. 

Mercury    87         Yesta    1,325  Jupiter  4,332 

Venus      224         Juno      1,592  Saturn  10,759 

Earth      365      .    Ceres     1,681  Herschel  30,686 

Mars       686         Pallas    1,686  iS'eptune  unknown."^ 

3.  The  mean  distancej  of  each  of  the  planets  from  the  sun 
is  expressed  as  follows,  in  millions  of  miles. 

Millions.                       JNIillions  Millions 

Mercury    36         Vesta       225          Jupiter  495 

Venus       68         Juno        254         Saturn  908 

Earth        95         Ceres       263          Herschel  1,827 

Mars        145         Pallas      264         JS^eptune  — 

4.  While  the  planets  revolve  around  the  sun,  each  also 
turns  around  upon  its  own  axis,  and  thus  presents  each  side 
successively  to  the  sun. 

5.  The  time  in  which  they  turn  upon  their  axes  is  called  their 
day,  and  is  thus  expressed  in  hours  and  minutes  of  om'  time. 

H.     M.  H.  H.  M. 

Mercury  24    5    Vesta      (unJcnown.)    Jupiter    9  55 
Venus    23  20   Juno  21  {2^rohahly.)    Saturn   10  16 
Earth     23  56    Ceres      {unknown.)    ^QY^chel  (unknown,) 
Mars      24  39    Pallas     {unknown.)    ISTeptune  — 
The  sun  turns  on  its  axis  in  about  25  days  anJ  10  hours. 

*  The  elements  of  this  planet  are  not  yet  sufficiently  determined  to  give 
them  with  any  degree  of  certainty. 

t  The  paths  or  orbits  of  the  planets  are  not. exactly  circular,  but  ellipti- 
cal. They  are,  therefore,  sometimes  nearer  to  the  sun  than  at  others. 
The  mean  distance  is  the  medium  between  their  greatest  and  least  dis- 
tance. Those  planets  which  are  nearer  to  the  sun  than  the  earth  are 
called  inferior  planets,  because  their  orbits  are  within  that  of  the  earth  ; 
and  those  which  are  farther  from  the  sun  are  called  superior  planets,  be- 
cause their  orbits  are  outside  that  of  the  earth. 


Note.  Of  what  form  are  the  orbits  of  the  planets  ?  What  is  meant  by 
the  mean  distance ?  What  planets  are  called  inferior?  Why?  What 
planets  are  called  superior?  Why?  What  is  the  distance  of  the  planet 
Mercury  from  the  sun ?  Venus?  Earth?  Mars?  Vesta?  Juno?  Ceres? 
Pallas  ?  Jupiter  ?  Saturn  ?  Herschel  ?  Neptune  ?  Have  the  planets 
any  motion  besides  that  around  the  sun  ?  What  is  the  time  in  which  they 
turn  upon  their  axes  called?  What  is  the  length  of  a  day  on  the  planet 
Mercury?  Venus?  Earth?  Mars?  Vesta?  Juno?  Ceres?  Pallas? 
Jupiter?    Saturn?    Herschel?  Neptune? 


ASTRONOMY. 


2(U 


307.  The  relative  size  of  the  bodies  belonging  to  the 
solar  system,  as  expressed  by  the  length  of  their  diame- 
ters, is  as  follows : 


The  Sun 
Mercury 
Venus 
Earth 
The  Moon 


Miles. 

877,547 
2,984 
7,621 
7,924 
2,180 


Mars 
Yes  La 
Juno 
Ceres 


Miles. 
4,222 
269 
1,393 
1,582 


Pallas 

Jupiter 

Saturn 


Miles. 
2,025 
86,255 


Herschel  34,363 


Fig.  151  is  a  representation  of  the  comparative  size  of  the 
planets."^ 

Fig.  151. 


Herschel 


*  Sir  J.  F.  W.  Herschel  gives  the  following  illustration  of  the  compara- 
tive size  and  distance  of  the  bodies  of  the  solar  system :  "  On  a  vi^ell -lev- 
elled field  place  a  globe,  two  feet  in  diameter,  to  represent  the  Sun  ;  Mer- 
cury will  be  represented  by  a  grain  of  mustard-seed  on  the  circumference 
of  a  circle  164  feet  in  diameter  for  its  orbit ;  Venus,  a  pea,  on  a  circle  284 
feet  in  diameter  ;  the  Earth,  also  a  pea,  on  a  circle  of  430  feet ;  Mars,  a 
rather  large  pin's  head,  on  a  circle  of  654  feet ;  Juno,  Ceres,  Vesta,  and 
Pallas,  grains  of  sand,  in  orbits  of  from  1000  to  1200  feet ;  Jupiter,  a  mod- 
erate-sized orange,  in  a  circle  nearly  half  a  mile  in  diameter;  Saturn,  a 
small  orange,  on  a  circle  of  four-fifths  of  a  mile  in  diameter  ;  and  Herschel 
a  full-sized  cherry,  or  small  plum,  upon  the  circumference  of  a  circle  more 
than  a  mile  and  a  half  in  diameter. 

"  To  imitate  the  motions  of  the  planets  in  the  above-mentioned  orbits, 

307.  What  is  the  diameter  of  the  Sun  ?  Mercury  ?  Venus  ?  Earth  ? 
Mars?  Vesta?  Juno?  Ceres?  Pallas?  Jupiter?  Saturn?  Herschel? 
The  Moon?  What  does  Fig.  151  represent?  What  illustration  of  the 
comparative  size  and  distance  of  the  bodies  of  the  solar  system  is  given? 
What  is  necessary  in  order  to  imitate  the  motions  of  the  planets  in  the 
above-mentioned  orbits  ? 


262 


NATURAL  PHILOSOPHY. 


308.  The  ecliptic  is  the  apparent  path  of  the  sun,  or 
the  real  path  of  the  earth. 

It  is  called  the  ecliptic,  because  every  eclipse,  whether  of 
the  sun  or  the  moon,  must  be  upon  it. 

309.  The  zodiac  is  a  space  or  belt,  16  degrees  broad, 
8  degrees  each  side  of  the  ecliptic. 

It  is  called  the  zodiac,  from  a  Greek  word,  which  signifies 
an  animal,  because  all  the  stars  in  the  twelve  parts  into  which 
the  ancients  divided  it,  were  formed  into  constellations,  and 
most  of  the  twelve  constellations  were  called  after  some  ani- 
mal.^ 

310.  The  zodiac  is  divided  into  twelve  signs,f  each 
sign  containing  thirty  degrees  of  the  great  celestial  cir- 

Mercury  must  describe  its  own  diameter  in  41  seconds  ;  Venus  in  4  min- 
utes and  14  seconds ;  the  Earth  in  7  minutes ;  Mars  in  4  minutes  and  48 
seconds;  Jupiter  in  2  hours  56  minutes;  Saturn  in  3  hours  13  minutes: 
and  Herschel  in  12  hours  16  minutes." 

*  Sir  J.  F.  W.  Herschel,  in  his  excellent  treatise  on  Astronomy,  says 
"  Uncouth  figures  and  outlines  of  men  and  monsters,  are  usually  scribbled 
over  celestial  globes  and  maps,  and  serve,  in  a  rude  and  barbarous  way,  to 
enable  us  to  talk  of  groups  of  stars,  or  districts  in  the  heavens,  by  names 
which,  though  absurd  or  puerile  in  their  origin,  have  obtained  a  currency, 
from  which  it  would  be  difficult,  and  perhaps  wrong,  to  dislodge  them.  In 
so  far  as  they  have  really  (as  some  have)  any  slight  resemblance  to  the 
figures  called  up  in  imagination  by  a  view  of  the  more  splendid  '  constel- 
lations,' they  have  a  certain  convenience ;  but  as  they  are  otherwise  en- 
tirely arbitrary,  and  correspond  to  no  natural  subdivisions  or  groupings  of 
the  stars,  astronomers  treat  them  lightly,  or  altogether  disregard  them,  ex- 
cept for  briefly  naming  remarkable  stars  ;  as  •  Alpha  Leonis,'  '  Beta  Scor- 
pii,'  &c.,  by  letters  of  the  Greek  alphabet  attached  to  them. 

"  This  disregard  is  neither  supercilious  nor  causeless.  The  constellations 
seem  to  have  been  almost  purposely  named  and  delineated  to  cause  as 
much  confusion  and  inconvenience  as  possible.  Innumerable  snakes  twine 
through  long  and  contorted  areas  of  the  heavens,  where  no  memory  can 
follow  them  ;  bears,  lions,  and  fishes,  large  and  small,  northern  and  south- 
ern, confuse  all  nomenclature,  &.c.  A  better  system  of  constellations 
might  have  been  a  material  help  as  an  artificial  memory." 

t  The  signs  of  the  Zodiac  and  the  various  bodies  of  the  Solar  system, 
are  often  represented  in  almanacs  and  astronomical  works,  by  signs  or 


308.  What  is  the  ecliptic  ?    Why  is  it  called  the  ecliptic  ? 

309.  What  is  the  zodiac  ?    Why  is  it  called  the  zodiac  ^ 


ASTRONOMY. 


203 


cle.*  The  names  of  these  signs  are  sometimes  given  in 
Latin,  and  sometimes  in  English.    They  are  as  follows  : 

Latin.  English.  Latin.  English. 

1  Aries,  The  Kam.         1  Libra,  The  Balance. 

2  Taurus,  Tlie  Bull.  8  Scorpio,        The  Scorpion. 

3  Gemini,  Tlie  Twins.        9  Sagittarius,    The  Archer. 

4  Cancer,  The  Crab.  10  Capricornus,  The  Goat. 

5  Leo,  The  Lion.  1 1  Aquarius,      The  Water-bearer. 

6  Yirgo,  The  Virgin.  12  Pisces,  The  Fishes. 

311.^  The  orbits  of  the  other  planets  are  inclined  to 
that  of  the  earth  ;  or,  in  other  words,  they  are  not  in  the 
same  plane. 

1.  Fig.  152  represents  an  oblique  view  of  the  plane  of  the 

characters.  In  the  following  list  the  characters  of  the  planets,  &c.,  are 
represented. 

0  The  Sun.  0  The  Earth.  ^  Ceres, 

d  The  Moon.  ^  Mars.  $  Pallas. 

^   Mercury.  g  Vesta.  U  Jupiter. 

2   Venus.  5  Juno.  Saturn. 

1^1  Herschel. 

The  following  characters  represent  the  signs  of  the  Zodiac. 

T  Aries.  ^  Leo.  /  Sagittarius, 

b  Taurus.  ]JJl  Virgo.  V3  Capricornus. 

n  Gemini.  d:^  Libra.  Aquarius. 

S  Cancer.  TTl  Scorpio.  ^  Pisces. 

From  an  inspection  of  Fig.  152  it  appears,  that  when  the  earth,  as  seen 
from  the  sun,  is  in  any  particular  constellation,  the  sun,  as  viewed  from 
the  earth,  will  appear  in  the  opposite  one. 

*  The  constellations  of  the  zodiac  do  not  now  retain  their  original 
names.  Each  constellation  is  about  30  degrees  eastward  of  the  sign  of 
the  same  name.  For  example,  the  constellation  Aries  is  30  degrees  east- 
ward of  the  sign  Aries,  and  the  constellation  Taurus  30  degrees  eastward 
of  the  sign  Taurus,  and  so  on.  Thus  the  sign  Aries  lies  in  the  constella- 
tion Pisces  ;  the  sign  Taurus  in  the  constellation  Aries  ;  the  sign  Gemini 
in  the  constellation  Taurus,  and  so  on.  Hence  the  importance  of  distin- 
guishing between  the  signs  of  the  zodiac  and  the  constellations  of  the  zo- 
diac. The  cause  of  the  difFerence  is  the  precession  of  the  equinoxes,  a 
phenomenon  which  will  be  explained  in  its  pro^^-^"  connexion. 

310.  What  are  the  names  of  the  twelve  constellations?  How  many 
degrees  does  each  sign  contain? 

311.  Are  the  orbits  of  the  other  planets  in  the  same  plane  with  that  of 
the  earth  ? 


264 


NATURAL  PHILOSOPHY. 


ecliptic,  the  orbits  of  all  the  primary  planets,  and  of  the  comet 
of  1680.    That  part  of  each  orbit  which  is  above  the  plane  is 

Fig.  152. 


shown  by  a  white  line ;  that  which  is  below  it,  by  a  dark  line. 
That  part  of  the  orbit  of  each  planet  where  it  crosses  the 


ASTRONOMY. 


Fig.  153. 


ecliptiC;  or,  in  other  words,  where  the  white  and  dark  lines  in 
tlie  figure  meet,  are  called  the  nodes  of  the  planet,  from  the 
Latin  uodus,  a  knot  or  tie. 

2.  Fig.  153  represents  a  section  of  the  plane 
of  the  ecliptic,  showing  the  inclination  of  the  or- 
bits of  the  planets.  As  the  zodiac  extends  only 
eight  degrees  on  each  side  of  the  ecliptic,  it  ap- 
pears from  the  figm-e  that  the  orbits  of  some  of  the 
planets  are  wholly  in  the  zodiac,  while  those  of 
others  rise  above  and  descend  below  it.  Thus, 
the  orbits  of  Juno,  Ceres,  and  Pallas,  rise  above, 
while  those  of  all  the  other  planets  are  confined 
to  the  zodiac. 

312.  When  a  planet  or  heavenly  body 
is  in  that  part  of  its  orbit  which  appears  to 
be  near  any  particular  constellation,  it  is 
said  to  be  in  that  constellation. 

Thus,  in  Fig.  152,  the  comet  of  1680  appears 
to  approach  the  sun  from  the  constellation  Leo. 

313.  The  perihelion*  and  aphelion*  of 
a  heavenly  body  express  its  situation  v^ith 
regard  to  the  sun.  When  a  body  is  nearest 
to  the  sun,  it  is  said  to  be  in  its  perihelion. 
When  farthest  from  the  sun,  it  is  said  to  be 
in  its  aphelion. 

The  earth  is  three  millions  of  miles  nearer  to 
the  sun  in  its  perihelion  than  in  its  aphelion. 

314.  The  apogee*  and  perigee*  of  a 

*  The  plural  of  Perihelion  is  Perihelia,  and  of  Aphe- 
lion is  Aphelia.  When  a  planet  is  so  nearly  on  a  line 
with  the  earth  and  the  sun  as  to  pass  between  them,  it 
is  said  to  be  in  its  inferior  conjunction ;  when  behind 
the  sun,  it  is  said  to  be  in  its  superior  conjunction ;  but 
when  behind  the  earth,  it  is  said  to  be  in  opposition. 


What  does  Fig.  152  represent?  What  are  the  nodes  of  a  planet?  What 
does  Fig.  153  represent? 

312.  When  is  a  planet  said  to  be  in  any  particular  constellation  ? 

313.  What  do  the  perihelion  and  aphelion  of  a  heavenly  body  express? 
When  is  a  body  said  to  be  in  its  perihelion  ?  When  is  a  body  said  to  be  in 
its  aphelion  ?  How  much  nearer  is  the  earth  to  the  sun  in  its  perihelion 
than  in  its  aphelion? 


266 


NATURAL  PHILOSOPHY. 


heavenly  body  express  its  situation  with  regard  to  the 
earth.  When  the  body  is  nearest  to  the  earth,  it  is  said  to 
be  in  perigee ;  when  it  is  farthest  from  the  earth,  it  is 
said  to  be  in  apogee. 

315.  The  periheUa  of  the  planets  are  in  the  following 
signs  of  the  zodiac  : — Mercury  in  Sagittarius, — Venus 
in  Aquarius^ — the  Earth  in  Capricornus, — Mars  in 
Virgo, — Vesta  in  Cancer, — Juno  in  Scorpio, — Ceres 
in  Pisces, — Pallas  in  Aquarius, — Jupiter  in  Libra, — 
Saturn  in  Capricornus, — and  the  Georgium  Sidus  in 
Aries,  ^ 

316.  The  axes  of  the  planets  in  their  revolution 
around  the  sun,  are  not  perpendicular  to  their  orbits, 
nor  to  the  plane  of  the  ecliptic,  but  are  inclined  in 
different  degrees. 

This  is  one  of  the  most  remarkable  circumstances  in 
the  science  of  Astronomy,  because  it  is  the  cause  of  the 
different  seasons,  spring,  summer,  autumn,  and  winter ; 
and  because  it  is  also  the  cause  of  the  difference  in  the 
length  of  the  days  and  nights  in  the  different  parts  of  the 
world,  and  at  the  different  seasons  of  the  year. 

317.  The  motion  of  the  heavenly  bodies  is  not  uni- 
form.   They  move  with  the  greatest  velocity  when 

The  words  perihelion,  aphelioii,  apogee,  and  perigee,  are  derived  from  the 
Greek  language,  and  have  the  foilowmg  meaning: 

Perihelion,  near  the  sun. 

ApheUon,  from  the  sun. 

Perigee,  near  the  earth. 

Apogee,  from  the  earth. 

Note.  When  is  a  planet  said  to  be  in  its  inferior  conjunction?  When 
is  it  said  to  be  in  its  superior  conjunction  ?  When  is  it  said  to  be  in  opposi- 
tion ? 

314.  What  do  the  apogee  and  perigee  of  a  heavenly  body  express  ?  When 
is  a  body  said  to  be  in  its  perigee?    When  is  it  said  to  be  in  its  apogee  ? 

315.  In  what  sign  is  the  perihelion  of  the  planet  Mercury?  Venus? 
Earth?  Mars?  Vesta?  Juno?  Ceres?  Pallas?  Jupiter?  Saturn? 
Georgium  Sidus? 

316.  What  is  said  with  regard  to  the  axes  of  the  planets  in  their  revolu- 
tion around  the  sun  ?    What  does  this  inclination  of  their  axes  cause  ? 

317.  What  is  said  with  regard  to  the  motion  of  the  heavenly  bodies? 


ASTRONOMY. 


267 


they  are  in  perihelion^  or  in  that  part  of  their  orbit 
which  is  nearest  to  the  sun ;  and  slowest  when  in 
aphelion. 

318.  It  has  been  proved  by  Kepler,  that  when  a  body 
moves  around  a  point  to  which  it  is  attracted,  a  line* 
drawn  from  the  point  to  the  body  passes  over  or  describes 
equal  areas  in  equal  times.    This  is  called  Kepler's  law. 

In  Fig.  154,  let  S  represent  the  sun,  and  E  the  earth,  and 
the  ellipse,  or  oval,  be  the  earth's  orbit,  or  path  around  the 
sun.  By  lines  drawn  from  the  sun  at  S  to  the  outer  edge 
of  the  figure,  the  orbit  is 
divided  into  twelve  areas 
of  ditferent  shapes,  but 
each  containing  the  same 
quantity  of  space.  Thus, 
the  spaces  E  S  A,  A  S  B, 
DS  F,  &c.,  are  all  sup- 
posed to  be  equal.  Now 
if  the  earth  in  the  space 
of  one  month,  will  move 
in  its  orbit  from  E  to  A, 
it  will,  in  another  month, 
move  from  A  to  B,  and 
in  the  third  month  from 
B  to  C,  &c.,  and  thus 
will  describe  equal  areas 
in  equal  times. 

The  reason  why  the 
earth  (or  any  other  heav- 
enly body)  moves  with  a  greater  degree  of  velocity  in  its 
perihelion,  than  in  its  aphelion,  may  likewise  be  explained  by 
the  same  figure.    Thus, — 

The  earth,  in  its  progress  from  F  to  L,  being  constantly  urged 
forward  by  the  sun's  attraction,  must  (as  is  the  case  with  a 
falling  body)  move  with  an  accelerated  motion.    At  L,  the 

*  This  line  is  called  the  radius-vector. 


When  do  they  move  with  the  greatest  velocity  ?  When  is  their  motion 
the  slowest  ? 

318.  What  is  Kepler's  law?  Illustrate  this  by  Fig.  154.  Explain,  by 
Fig  154,  the  reason  w^hy  the  earth,  or  any  other  heavenly  body,  moves 
with  a  greater  degree  of  velocity  in  its  perihelion  than  in  its  aphelion. 


268 


NATURAL  PHILOSOPHY. 


sun's  attraction  becomes  stronger,  on  account  of  the  nearness 
of  the  earth ;  and,  consequently,  in  its  motion  from  L  to  E,  the 
earth  will  move  with  greater  rapidity.  At  E,  which  is  the 
perihelion  of  the  earth,  it  acquires  its  greatest  velocity.  Let 
us  now  detain  it  at  E,  merely  to  consider  the  direction  of  the 
forces  by  which  it  is  urged.  If  the  sun's  attraction  could 
be  destroyed,  the  force  which  has  carried  it  from  L  to  E, 
would  carry  it  off  in  the  dotted  line  from  E  to  G,  wliich  is  a 
tangent  to  its  orbit.  But  while  the  earth  has  this  tendency  to 
move  towards  G,  the  sun's  attraction  is  continually  operating 
with  a  tendency  to  carry  it  to  S.  Now,  when  a  body  is 
urged  by  two  forces,  it  will  move  between  them ;  but  as  the 
sun's  attraction  is  constantly  exerted,  the  direction  of  the 
earth's  motion  will  not  be  in  a  straight  line,  the  diagonal  of 
one  large  parallelogram,  but  through  the  diagonal  of  a  number 
of  infinitely  small  parallelograms ;  which,  being  united,  form 
the  curve  line  E  A. 

It  is  thus  seen,  that,  while  the  earth  is  moving  from  L  to  E, 
the  attraction  of  the  sun  is  stronger  than  in  any  other  part  of 
its  orbit,  and  will  cause  the  earth  to  move  rapidly.  But  in  its 
motion  from  E  to  A,  from  A  to  B,  from  B  to  C,  and  from  C 
to  F,  the  attraction  of  the  sun,  operating  in  an  opposite  direc- 
tion, will  cause  its  motion  from  the  sun  to  be  retarded,  until,  at 
F,  the  direction  of  its  motion  is  reversed,  and  it  begins  again 
to  approach  the  sun.  Thus,  it  appears  that,  in  its  passage 
from  the  perihelion  to  the  aphehon,  the  motion  of  the  earth,  as 
well  as  that  of  all  the  heavenly  bodies,  must  be  constantly  re- 
tarded ;  while,  in  moving  from  their  aphelion  to  perihelion,  it  is 
constantly  accelerated  ;  and  at  their  perihehon,  the  velocity 
will  be  the  greatest.  The  earth,  therefore,  is  about  seven  days 
longer  in  performing  the  aphehon  part  of  its  orbit,  than  in 
traversing  the  perihehon  part ;  and  the  revolution  of  all  the 
other  planets  being  the  result  of  the  same  cause,  is  affected  in 
the  same  manner  as  that  of  the  earth. 

319.  The  earth  is  about  three  millions  of  miles  nearer 
to  the  sun  in  winter  than  in  summer. 


What  is  said  of  the  motion  of  the  heavenly  bodies  from  perihelion  to 
aphelion  ?  What  is  their  motion  from  aphelion  to  perihelion  ?  When  is 
their  velocity  the  greatest?  How  much  longer  is  the  earth  in  performing 
the  aphelion  part  of  its  orbit  than  the  perihelion  part  ? 

319.  How  much  nearer  is  the  earth  to  the  sun  in  winter  than  in  sum- 
mer ? 


ASTRONOMY. 


269 


The  licat  of  suininer,  therefore,  cannot  be  caused  by 
the  near  approach  of  the  earth  to  the  sun. 

Snow  and  ice  never  melt  on  the  tops  of  high  mountains  ;  and 
they  who  have  ascended  in  the  atmosphere,  in  balloons,  have 
fomid  that  the  cold  increases  as  they  rise. 

320.  On  account  of  the  inclination  of  the  earth's  axis, 
the  rays  of  the  sun  fall  more  or  less  obliquely  on  differ- 
ent parts  of  the  earth's  surface,  at  different  seasons  of 
the  year.  The  heat  is  always  the  greatest  when  the 
sun's  rays  fall  vertically ;  and  the  more  obliquely  they 
fall,  the  less  heat  they  appear  to  possess. 

1.  This  is  the  reason  why  the  days  are  hottest  in  summer, 
although  the  earth  is  farther  from  the  sun  at  that  time. 

2.  Fig.  155  represents  the  manner  in  which  the  rays  of  the 
sun  fall  upon  the  earth  in  summer  and  in  winter.  The  north 
pole  of  the  earth,  at  all  seasons,  constantly  points  to  the  north 

Fig.  155. 


star  N ;  and  when  the  earth  is  nearest  to  the  sun,  the  rays 
from  the  sun  fall  as  indicated  by  W,  in  the  figure ;  and  as 
their  direction  is  very  oblique,  and  they  have  a  larger  portion 


320.  What  follows  from  the  mclination  of  the  earth's  axis,  with  regard 
to  the  direction  of  the  sun's  rays?  When  is  the  heat  always  the  greatest? 
What  is  said  of  oblique  rays?  What  is  the  reason  that  the  heat  is  greater 
in  summer  than  in  winter?  Illustrate  this  by  Fig.  155.  How  i»  the  earth 
situated  with  regard  to  its  distance  from  the  sun  in  winter  I 


270 


MATURAL  PHILOSOPHY. 


of  the  atmospliere  to  traverse,  much  of  their  power  is  lost. 
Hence  we  have  cold  weather  when  the  earth  is  nearest  to  the 
sun.  But  when  the  earth  is  in  aphehon,  the  rays  fall  almost  ver- 
tically or  perpendicularly,  as  represented  by  S,  in  the  figure ; 
and,  although  the  earth  is  then  nearly  three  millions  of  miles 
farther  from  the  sun,  the  heat  is  greatest,  because  the  rays  fall 
more  directly,  and  have  a  less  portion  of  the  atmosphere  to 
traverse.* 

3.  For  a  similar  reason,  we  find,  even  in  summer,  that  early 
'n  the  morning,  and  late  in  the  afternoon,  it  is  much  cooler 
than  at  noon,  because  the  sun  then  shines  more  obliquely. 
The  heat  is  generally  the  greatest  at  about  three  o'clock  in 
the  afternoon;  because  the -earth  retains  its  heat  for  some 
length  of  time,  and  the  additional  heat  it  is  constantly  re- 
ceiving from  the  sun,  causes  an  elevation  of  temperature,  even 
after  the  rays  begin  to  fall  more  obliquely. 

It  is  the  same  cause  which  occasions  the  variety  of  chmate 
in  different  parts  of  the  earth.  The  sim  always  shines  in  a 
direction  nearly  perpendicular,  or  vertical,  on  the  equator; 
and  with  different  degrees  of  obhquity  on  the  other  parts  of 
the  earth.  For  this  reason,  the  greatest  degree  of  heat  pre- 
vails at  the  equator  during  the  whole  year.  The  fajlher  any 
place  is  situated  from  the  equator,  the  more  obliquely  will  the 
rays  fall,  at  different  seasons  of  the  year ;  and,  consequently, 
the  greater  will  be  the  difference  in  the  temperature. 

4.  If  the  axis  of  the  earth  were  perpendicular  to  its  orbit, 
those  parts  of  the  earth  which  lie  under  the  equator  would  be 
constantly  opposite  to  the  sun ;  and  as,  in  that  case,  the  sun 
would,  at  all  times  of  the  year,  be  vertical  to  those  places 
equally  distant  from  both  poles,  so  the  light  and  heat  of  the 
sun  would  be  dispersed  with  perfect  uniformity  towards  each 

*  This  may  be  more  familiarly  explained,  by  comparing  summer  rays 
to  a  ball  or  stone  thrown  directly  at  an  object,  so  as  to  strike  it  with  all  its 
force ;  and  winter  rays  to  the  same  ball  or  stone,  thrown  obliquely,  so  as 
merely  to  graze  the  object. 

What  illustration  of  oblique  and  perpendicular  rays  is  given  in  the  note? 
Why  is  it  generally  cooler  early  in  the  morning  and  late  in  the  afternoon 
than  at  noon?  Why  is  the  heat  the  greatest  at  about  three  o'clock? 
What  causes  the  variety  of  climate  in  different  parts  of  the  earth  ?  Where 
does  the  sun  always  shine  in  a  vertical  direction  ?  What  would  follow 
were  the  axis  of  the  earth  perpendicular  to  its  orbit  ?  What  causes  the 
variety  of  the  seasons,  the  different  lengths  of  days  and  nights,  ? 


ASTRONOMY. 


271 


pole ;  we  should  have  no  variety  of  seasons ;  day  and  night 
would  be  of  the  same  length ;  and  the  heat  of  the  sun  would 
be  of  the  same  intensity  every  day  throughout  the  year. 

5.  It  is,  therefore,  as  has  been  stated,  owing  to  the  inclina- 
tion of  the  earth's  axis,  that  we  have  the  agreeable  variety  of  the 
seasons,  days  and  nights  of  different  lengths,  and  that  wisely- 
ordered  variety  of  climate,  which  causes  so  great  a  variety  of 
productions,  and  which  has  afforded  so  powerful  a  stimulus  to 
human  industry.^ 

6.  In  order  to  understand  the  illustration  of  the  causes  of 
the  seasons,  &c.,  it  is  necessary  to  have  some  knowledge  of 
the  circles  which  are  drawn  on  .the  artificial  representations 
of  the  earth.  It  is  to^e  remembered  that  all  of  these  circles 
are  wholly  imaginary ;  that  is,  that  there  is  on  the  earth  itself 
no  such  circles  or  lines.  They  are  drawn  on  maps  merely  for 
the  purpose  of  illustration. 

Y.  Fig.  156  represents  the  earth.    N  S  is  the  axis,  or  ima- 
ginary hne,  around  which  it  daily  turns ;  N  is  the  north  pole, 
S  is  the  south  pole.    These  poles, 
it  will  be  seen,  are  the  extremities  Fig-  i^^- 

of  the  axis  N"  S.  CD  represents 
the  equator,  which  is  a  circle  around 
the  earth,  at  an  equal  distance  from 
each  pole.  The  curved  lines  pro- 
ceeding from  IST  to  S,  are  meridians. 
They  are  all  circles  surrounding  the 
earth,  and  passing  through  the 
poles.  These  meridians  may  be 
multiplied  at  pleasure. 

The  lines  E  F,  I K,  L  M,  and 
G  H,  are  designed  to  represent  cir- 

*  The  wisdom  of  Providence  is  frequently  displayed  in  apparent  incon- 
sistencies. Thus,  the  very  circumstance  which,  to  the  short-sighted 
philosopher,  appears  to  have  thrown  an  insurmountable  barrier  between 
the  scattered  portions  of  the  human  race,  has  been  wisely  ordered,  to  es- 
tablish an  interchange  of  blessings,  and  to  bring  the  ends  of  the  earth  in 
communion.  Were  the  same  productions  found  in  every  region  of  the 
earth,  the  stimulus  to  exertion  would  be  weakened,  and  the  wide  field  of 
human  labor  would  be  greatly  diminished.  It  is  our  mutual  wants  which 
bind  us  together. 


What  is  necessary  in  order  to  understand  the  illustration  of  the  causes 
of  the  seasons?    Explain  Fig.  156.    What  are  the  poles? 


272  , 


NATURAL  PHILOSOPHY. 


cles,  all  of  them  parallel  to  the  equator,  and  for  this  reason 
they  are  called  parallels  of  latitude.  These  also  may  be 
multiplied  at  pleasure. 

But  in  the  figure,  these  lines,  which  are  parallel  to  the 
equator,  and  which  are  at  a  certain  distance  from  it,  have  a 
different  name,  derived  from  the  manner  in  which  the  sun's 
rays  fall  on  the  surface  of  the  earth. 

Thus  the  circle  IK,  23-^-  degrees  from  the  equator,  is  called 
the  tropic  of  Cancer,'^  and  the  circle  L  M  is  called  the  tropic 
of  Capi  icorn.  The  circle  E  F  is  called  the  Arctic  Circle.  It 
represents  the  limit  of  perpetual  day,  when  it  is  summer  in 
the  northern  hemisphere,  and  of  perpetual  night  when  it  is 
winter. 

The  circle  G  H  is  the  Antarctic  Circle,  and  represents  the 
limit  of  perpetual  day  and  night  in  the  southern  hemisphere. 
The  line  L  K  represents  the  circle  of  the  ecliptic,  which,  as 
has  already  been  stated,  is  the  apparent  path  of  the  sun,  or 
the  real  path  of  the  earth.  This  circle,  although  it  is  gener- 
ally drawn  on  the  terrestrial  globe,  is,  in  reality,  a  circle  in 
the  heavens ;  and  differs  from  the  zodiac  only  in  its  width, 
— the  zodiac  extending  eight  degrees  on  each  side  of  the 
ecliptic. 

8.  Fig.  157  represents  the  manner  in  which  the  sun  shines 
on  the  earth  in  different  parts  of  its  orbit ;  or,  in  other  words, 
the  cause  of  the  change  in  the  seasons.  S  represents  the  sun, 
and  the  dotted  oval,  or  ellipse,  A  B  C  D,  the  orbit  of  the 

*  Thus  on  the  21st  of  March,  the  rays  of  the  sun  fall  vertically  on  the 
equator,  and  on  each  succeeding  day  on  places  a  little  to  the  north,  until 
the  21st  of  June,  when  they  fail  vertically  on  places  23^  degrees  north  of 
the  equator.  Their  vertical  direction  then  turns  back  again  towards  the 
equator,  where  the  rays  again  fall  vertically  on  the  23d  of  September,  and 
on  the  succeeding  days  a  little  to  the  south,  until  the  21st  of  December, 
when  they  fall  vertically  on  the  places  23  J  south  of  the  equator.  Their 
vertical  direction  then  again  turns  towards  the  equator.  Hence  the  cir- 
cles I  K  and  L  M  are  called  the  tropics  of  Cancer  and  Capricorn.  The 
word  tropic  is  derived  from  a  word  which  signifies  to  turn.  The  tropics, 
therefore,  are  the  boundaries  of  the  sun's  apparent  path  north  and  south  of 
the  equator,  or  the  lines  at  which  the  sun  turns  back. 

Why  is  the  circle  I  K  called  the  tropic  of  Cancer?  What  is  the  mean- 
ing of  the  word  tropic  ?  Why  is  the  circle  L  M  called  the  tropic  of  Capri- 
corn ?  What  are  the  tropics  ?  What  is  the  circle  E  F  called  ?  What 
does  it  represent  ?  What  is  the  circle  G  H  called  ?  What  does  it  repre- 
sent ? 


ASTRONOMY. 


273 


earth.  The  outer  circle  represents  the  zodiac,  with  the  posi- 
tion of  the  twelve  signs  or  constellations.    On  the  21st  of 


Fig.  157. 


June,  when  the  earth  is  at  D,  the  whole  northern  polar  region 
is  continually  in  the  light  of  the  sun.  As  it  turns  on  its  axis, 
therefore,  it  will  be  day  to  all  the  parts  which  are  exposed  to 
the  light  of  the  sun.^    But,  as  the  whole  of  the  Antarctic 

*  Day  and  night  are  caused  by  the  rotation  of  the  earth  on  its  axis  every 
24  hours.  It  is  day  to  that  side  of  the  earth  which  is  towards  the  sun,  and 
night  to  the  opposite  side.  The  length  of  the  days  is  in  proportion  to  the 
inchnation  of  the  axis  of  the  earth  towards  the  sun.  It  may  be  seen  by 
tile  above  figure,  that  in  summer  the  axis  is  most  inchned  towards  the 
sun,  and  then  the  days  are  the  longest.  As  the  north  pole  becomes  less 
inclined,  the  days  shorten,  till,  on  the  21st  of  December,  it  is  inchned 


What  does  Fig.  157  represent?  Explain  the  figure.  Explain,  by  the 
figure,  the  situation  of  the  earth  on  the  21st  of  June.  What  causes  day 
and  night?  To  what  part  of  the  earth  is  it  day?  To  what  part  is  it 
night?  To  what  is  the  length  of  the  day  in  proportion?  When  are  the 
days  the  longest?    Why  ?    When  are  they  the  shortest?    Why  ? 

12* 


274 


NATURAL  PHILOSOPHY. 


Circle  is  within  the  Hne  of  perpetual  darkness,  the  sun  can 
shine  on  no  part  of  it.  It  will,  therefore,  be  constant  night  to 
all  places  within  that  circle.  As  the  whole  of  the  Arctic 
Circle  is  within  the  line  of  perpetual  light,  no  part  of  that 
circle  will  be  turned  from  the  sun  while  the  earth  turns  on  its 
axis.  To  all  places,  therefore,  within  the  Arctic  Circle,  it  will 
be  constant  day. 

On  the  2 2d  of  September,  when  the  earth  is  at  C,  its  axis 
is  neither  inclined  to  nor  from  the  sun,  but  is  sidewise ;  and, 
of  course,  while  one  half  of  the  earth,  from  pole  to  pole,  is 
enlightened,  the  other  half  is  in  darkness,  as  would  be  the 
case  if  its  axis  were  perpendicular  to  the  plane  of  its  orbit ; 
and  it  is  this  which  causes  the  days  and  nights,  of  this  season 
of  the  year,  to  be  of  equal  length. 

On  the  23d  of  December,  the  earth  has  progressed  in  its 
orbit  to  B,  which  causes  the  whole  space  within  the  northern 
polar  circle  to  be  continually  in  darkness,  and  more  of  that 
part  of  the  earth  north  of  the  equator  to  be  in  the  shade  than 
in  the  light  of  the  sun.  Hence,  on  the  21st  of  December,  at 
all  places  north  of  the  equator,  the  days  are  shorter  than  the 
nights,  and  at  all  places  south  of  the  equator,  the  days  are 
longer  than  the  nights.  Hence,  also,  within  the  Arctic  Circle 
it  is  uninterrupted  night,  the  sun  not  shining  at  all ;  "and  within 
the  Antarctic  Circle  it  is  uninterrupted  day,  the  sun  shining 
all  the  time. 

On  the  20th  of  March,  the  eartli  has  advanced  still  further, 
and  is  at  A,  which  causes  its  axis,  and  the  length  of  the  days, 
and  nights,  to  be  the  same  as  on  the  20th  of  September."^ 

23^  degrees /ro7«  the  sun,  when  the  days  are  the  shortest.  Thus,  as  the 
earth  progresses  in  its  orbit,  after  the  days  are  the  sh-ortest,  it  changes  its 
indination  towards  the  sun,  till  it  is  again  inclined  as  in  the  longest  days 
in  the  summer 

*  As  the  difference  in  the  length  of  the  days  and  the  nights,  and  the 
change  of  the  seasons,  &c.,  on  the  earth,  is  caused  by  the  inclination  of 
the  earth's  axis,  it  follows  that  all  the  planets,  whose  axes  are  inclined, 
must  experience  the  same  vicissitude  ;  and  that  it  must  be  in  proportion  to 


Explain,  by  the  figure,  the  situation  of  the  earth  on  the  22d  of  Septem- 
Der.  On  the  23d  of  December.  On  the  20th  of  March.  What  follows 
from  the  changes  on  the  earth,  caused  by  the  inclination  of  the  earth's 
axis  ?  In  what  proportion  are  these  changes?  What  is  said  of  the  axis  of 
the  planet  Jupiter  ? 


ASTRONOMY. 


275 


9.  From  the  explanation  of  figure  157,  it  appears  that 
there  are  two  parts  of  its  orbit  in  wliicli  tlie  days  and  nights 
are  equal  all  over  the  earth.  These  points  are  in  the  sign  of 
Aries  and  Libra,  which  are  therefore  called  the  equinoxes. 
Aries  is  the  vernal  (or  spring)  equinox,  and  Libra  the  autum- 
nal equinox. 

10.  There  are  also  two  other  points  called  solstices,  because 
the  sun  appears  to  stand  at  the  same  height  in  the  heavens,  in 
the  middle  of  the  day,  for  several  days.  These  points  are  in 
the  signs  Cancer  and  Capricorn.  Cancer  is  called  the  summer 
solstice,  and  Capricorn  the  winter  solstice. 

32 L  The  sun  is  a  spherical  body,  situated  near  the 
centre  of  gravity  of  the  system  of  planets  of  which  our 
earth  is  one. 

I.  Its  diameter  is  877,547  English  miles ;  which  is  equal  to 
100  diameters  of  the  earth;  and,  therefore,  his  cubic  magni- 
tude must  exceed  that  of  the  earth  one  milhon  of  times.  ^  It 

the  degree  of  the  inclination  of  their  axes.  As  the  axis  of  the  planet  Ju- 
piter is  nearly  perpendicular  to  its  orbit,  it  follows  that  there  can  be  little 
variation  in  the  length  of  the  days,  and  little  change  in  the  seasons  of  that 
planet. 

There  can  be  little  doubt  that  the  sun,  the  planets,  stars,  &c.,  are  all  of 
them  inhabited  ;  and  although  it  may  be  thought  that  some  of  them,  on 
account  of  their  immense  distance  from  the  sun,  experience  a  great  want 
of  light  and  heat,  while  others  are  so  near,  and  the  heat  consequently  so 
great  that  water  cannot  remain  on  them  in  a  fluid  state,  yet  as  we  see, 
even  on  our  own  earth,  that  creatures  of  different  natures  hve  in  different 
elementSj^  as,  for  instance,  fishes  in  water,  animals  in  air,  &c.,  creative 
wisdom  could,  undoubtedly,  adapt  the  being  to  its  situation,  and  with  as 
little  exertion  of  power,  form  a  race  whose  nature  should  be  adapted  to  the 
nearest  or  the  most  remote  of  the  heavenly  bodies,  as  was  required  to  adapt 
the  fowls  to  the  air,  or  the  fishes  to  the  sea. 

*  Spheres  are  to  each  other  as  the  cubes  of  their  respective  diameters. 


Note.  Is  it  supposed  that  the  sun,  planets,  and  stars  are  inhabited? 
What  is  shown  by  Fig.  157?  Where  are  these  points?  What  are  they 
called?  Which  is  the  vernal  equinox ?  Which  the  autumnal?  What 
other  two  points  are  there?  Why  are  they  called  solstices?  Where  are 
these  points  ?    Which  is  the  summer  solstice  ?    Which  the  winter  ? 

321.  What  is  said  of  the  sun  ?  What  is  its  diameter?  How  much  does 
its  cubic  magnitude  exceed  that  of  the  earth  ? 


276 


NATURAL  PHILOSOPHY. 


revolves  around  its  axis  in  25  days  and  10  hours.  This  has 
been  ascertained  by  means  of  several  dark  spots  which  have 
been  seen  with  telescopes  on  its  surface. 

2.  Dr.  Herschel  supposed  the  greater  number  of  spots  on 
the  sun  to  be  mountains ;  some  of  which  he  estimated  to  be 
300  miles  in  height. 

3.  It  is  probable  that  the  sun,*  like  all  the  other  heavenly 
bodies,  (excepting,  perhaps,  comets,)  is  inhabited  by  beings 
whose  nature  is  adapted  to  their  pecuhar  circumstances. 

4.  Although,  by  some,  the  sun  is  supposed  to  be  an  im- 
mense ball  of  fire,  on  account  of  the  effects  produced  at  the 
distance  of  ninety-five  milhons  of  miles,  yet  many  facts  show 
that  heat  is  produced  by  the  sun's  rays  only  when  they  act 
on  a  suitable  medium,  thus,  snow  and  ice  remain  during  the 
year  on  the  tops  of  the  highest  mountains,  even  in  chmates 
where  the  cold  of  our  winters  is  never  knoAvn. 

5.  The  zodiacal  light  is  a  singular  phenomenon,  accompany- 
ing the  sun.  It  is  a  faint  light  which  often  appears  to  stream 
up  from  the  sim  a  little  after  sunset  and  before  sunrise.  It 
appears  nearly  in  the  form  of  a  cone,  its  sides  being  somewhat 
curved,  and  generally  but  ill  defined.  It  extends  often  from 
50^  to  100^  in  the  heavens,  and  always  nearly  in  the  direc- 
tion of  the  plane  of  the  ecliptic.  It  is  most  distinct  about 
the  beginning  of  March ;  but  is  constantly  visible  in  the  torrid 
zone.    The  cause  of  this  phenomenon  is  not  known. 

322.  Mercury  is  the  nearest  planet  to  the  sun,  and  is 
seldom  seen  ;  because  his  vicinity  to  the  sun  occasions 
his  being  lost  in  the  brilliancy  of  the  sun's  rays. 

1.  The  heat  of  this  planet  is  so  great  that  water  cannot 
exist  there,  except  in  a  state  of  vapor ;  and  metals  would  be 
melted.  The  intenseness  of  the  sun's  heat,  which  is  in  the 
same  proportion  as  its  light,  is  seven  times  greater  in  Mercuiy 

*  In  almanacs,  the  sun  is  usually  represented  by  a  small  circle,  with 
the  face  of  a  man  in  it,  thus :  ^ 

How  long  is  it  in  performing  its  revolution  around  its  axis  ?  How  has 
this  been  ascertained  ?  What  did  Dr.  Herschel  suppose  these  spots  to  be  ? 
What  is  the  zodiacal  light  ?  At  what  time  is  it  most  distinct  ?  Where  is 
it  constantly  visible  ? 

322.  What  planet  is  nearest  to  the  sun  ?  Why  is  it  seldom  seen  ?  What 
is  said  of  the  heat  of  this  planet  ?  How  much  greater  is  the  suu's  heat  in 
Mercury  than  on  the  earth  ? 


ASTRONOMY. 


277 


tlian  on  the  earth ;  so  that  water  there  would  be  carried  off 
in  tlie  sliape  of  steam  :  for,  by  experiments  made  with  a  ther- 
m  meter,  it  appears  that  a  heat  seven  times  greater  than  that 
of  the  sun's  beams  in  summer,  will  make  water  boil. 

2.  Mercury,  although  in  appearance  only  a  small  star,  emits 
a  bright  white  light  by  which  it  may  be  recognised  when  seen. 
It  appears  a  httle  before  the  sun  rises,  and  again  a  little  after 
sunset,  but  as  its  angular  distance  from  the  sun  never  exceeds 
23  degrees,  it  is  never  to  be  seen  longer  than  one  hour  and 
fifty  minutes  after  sunset ;  nor  longer  than  that  time  before 
the  sun  rises. 

3.  When  viewed  through  a  good  telescope,  Mercury  ap- 
pears with  all  the  various  phases,  or  increase  and  decrease  of 
hght,  with  which  we  view  the  moon ;  except  that  it  never  ap- 
pears quite  full,  because  its  enlightened  side  is  turned  directly 
towards  the  earth  only  when  the  planet  is  so  near  the  sun  as 
to  be  lost  to  our  sight  in  its  beams.  Like  that  of  the  moon, 
the  crescent  or  enlightened  side  of  Mercury  is  always  towards 
the  sun.  As  no  spots  are  commonly  visible  on  the  disk,  the 
time  of  its  rotation  on  its  axis  is  unknown. 

323.  Venus,*  the  second  planet  in  order  from  the  sun, 
is  the  nearest  to  the  earth,  and  on  that  account  appears 
to  be  the  largest  and  most  beautiful  of  all  the  planets. 
During  a  part  of  the  year  it  rises  before  the  sun,  and  it 
is  then  called  the  morning  star  ;  during  another  part  of 
the  year  it  rises  after  the  sun,  and  it  is  then  called  the 
evening  star.  The  heat  and  light  at  Venus  are  nearly 
double  what  they  are  at  the  earth. 

*  By  the  ancient  poets,  Venus  was  called  Phosphor,  or  Lucifer,  when 
it  appeared  to  the  west  of  the  sun,  at  which  time  it  is  morning  star,  and 
ushers  in  the  light  or  day  ;  and  Hesperus  or  Vesper,  when  eastward  of 
the  sun,  or  evening  star. 


In  what  form  does  water  exist  in  Mercury  ?  How  can  Mercury  be 
recognised  when  seen  ?  At  what  time  does  it  appear  ?  How  does  Mer- 
cury appear  when  viewed  through  a  telescope  ? 

323.  What  planet  is  nearest  to  the  earth  ?  When  is  Venus  called  the 
morning  star?  When  is  it  called  the  evening  star?  How  much  greater 
are  the  light  and  heat  at  Venus  than  that  at  the  earth?  What  name  was 
given  by  the  ancient  poets  to  Venus,  when  morning  star?  What,  when 
evening  star? 


278 


NATURAL  PHILOSOPHY. 


1.  Yenus,  like  Mercury,  presents  to  us  all  the  appearances 
of  mcrease  and  decrease  of  light  common,  to  the  moon  Spots 
are  also  sometimes  seen  on  its  surface,  like  those  on 'the  sun 
By  reason  of  the  great  brilhancy  of  this  planet,  it  may  some- 
times be  seen  even  in  the  daytime,  by  the  naked  eye.*  But 
It  IS  never  seen  late  at  night,  because  its  angular  distance  from 
the  sun  never  exceeds  45  degrees.  In  the  absence  of  the 
moon  It  will  cast  a  shadow  behind  an  opaque  body. 

2.  Both  Mercury  and  Yenus  sometimes  pass  directly  be- 
tween the  sun  and  the  earth.  As  their  illuminated  surface  is 
towards  the  sun,  their  dark  side  is  presented  to  the  earth,  and 
they  appear  like  dark  spots  on  the  sun's  disk.  This  is  called 
the  transit  of  these  planets. 

324.  The  earth  is  the  next  planet,  in  the  solar  system, 
to  Venus.  It  is  not  a  perfect  sphere,  but  its  figure  is 
that  of  an  ohlate  spheroid,  the  equatorial  diameter  being 
about  34  miles  longer  than  its  polar  diameter. 

It  is  attended  by  one  moon,  the  diameter  of  which  is  about 
two  thousand  miles.  Its  mean  distance  from  the  earth  is 
about  240,000  miles,  and  it  turns  on  its  axis  in  precisely  the 
same  time  that  it  performs  its  revolution  round  the  earth; 
namely,  in  twenty-nine  days  and  a  half. 

325.  The  earth,  when  viewed  from  the  moon,  exhibits 
precisely  the  same  phases  that  the  moon  does  to  us,  hut 
in  opposite  order.  When  the  moon  is  full  to  us,  the 
earth  will  he  dark  to  the  inhabitants  of  the  moon  ;  'and 
when  the  moon  is  dark  to  us,  the  earth  will  he  full  to 

*  The  reason  why  we  cannot  see  the  stars  and  planets  in  the  daytime, 
is,  that  their  light  is  so  faint,  compared  with  the  light  of  the  sun  reflected 
by  our  atmosphere 


What  is  the  greatest  distance  at  which  the  planets,  Mercuiy  and  Venus, 
ever  appear  from  the  sun  ?  What  is  meant  by  the  transit  of  these  planets  ? 
What  is  said  of  the  different  appearances  which  Venus  presents?  Why 
can  we  not  see  the  planets  and  stars  in  the  daytime  ? 

324.  What  planet  is  next  to  Venus?  What  is  the  form  of  the  earth? 
How  much  larger  is  its  equatorial  diameter  than  its  polar?  How  many 
moons  has  the  earth  ?  What  is  the  diameter  of  the  moon  ?  What  is  its 
distance  from  the  earth  ?  What  is  the  length  of  a  day  at  the  moon  ?  How 
long  is  it  in  performing  its  revolution  around  the  earth  ? 

325.  What  phases  does  the  earth,  when  viewed  from  the  moon,  exhibit? 


ASTRONOMY. 


279 


them.  The.  earth  appears  to  them  about  18  times  larger 
than  the  moon  does  to  us.  As  the  moon,  however,  al- 
ways presents  nearly  the  same  side  to  the  earth,  there  is 
one-half  of  the  moon  which  we  never  see,  and  from  which 
the  earth  cannot  be  seen, 

3-26.  Next  to  the  earth  is  the  planet  Mars.  It  is  con- 
spicuous for  its  fiery  red  appearance  ;  which  is  supposed 
to  be  caused  by  a  very  dense  atmosphere. 

1.  When  this  planet  approaches  any  of  the  fixed  stars,  they 
change  their  color,  grow  dim,  and  often  become  totally  invisi- 
ble. '^This  is  supposed  to  be  caused  by  his  atmosphere.  The 
deo-ree  of  heat  and  hp^bt  at  Mars  is  less  than  half  of  that  re- 
ceived  by  the  earth. 

2.  The  four  small  planets,  or  asteroids,  Yesta,  Juno,  Ceres, 
and  Pallas,  have  all  been  discovered  within  the  present  cen- 
tury. Vesta  was  discovered  by  Dr.  Olbers,  of  Bremen,  in 
]  807  :  its  light  is  pure  and  white.  Juno,  by  Mr.  Harding, 
near  Bremen,  in  1804:  its  color  is  red,  and  its  atmosphere 
appears  cloudy.  Pallas  was  discovered  by  Dr.  Olbers  in 
1802  :  it  appears  to  have  a  dense,  cloudy  atmosphere.  Ceres 
was  discovered  at  Palermo,  in  Sicily,  by  Piazzi,  in  1801  :  it 
is  of  a  ruddy  color.  All  of  these  small  planets  undergo  va- 
rious changes  in  appearance  and  size,  so  that  their  real  magni- 
tude is  not  ascertained  with  any  certainty ;  and  but  httle  is 
known  of  them."^ 

*  It  is  a  remarkable  fact,  that  certain  irregularities,  observed  in  the 
motions  of  the  old  planets,  induced  some  astronomers  to  suppose  that  a 
planet  existed  between  the  orbits  of  Mars  and  Jupiter ;  a  supposition  that 
arose  long  previous  to  the  discovery  of  the  four  new  planets  just  noticed. 
The  opinion  has  been  advanced,  that  these  four  small  bodies  originally 
composed  one  larger  one,  which,  by  some  unknown  force  or  convulsion, 
burst  asunder.    This  opinion  is  maintained  with  much  ingenuity  and  plau- 

How  much  larger  does  the  earth  appear  than  the  moon? 

326.  What  planet  is  next  to  the  earth  ?  What  renders  it  conspicuous  ? 
What  is  supposed  to  cause  this  appearance  ?  How  much  more  light  and 
aeat  does  the  earth  enjoy  than  Mars  ?  When  were  the  asteroids  discov- 
ered? By  whom,  and  in  what  year  was  Vesta  discovered  ?  What  is  the 
iolor  of  its  light  ?  By  whom  and  when  was  Juno  discovered  ?  What  is 
ihe  color  of  its  light  ?  When  was  Pallas  discovered  ?  By  whom  ?  What 
^  said  of  its  atmosphere  ?  When  and  by  whom  was  Ceres  discovered  ? 
What  is  its  color?    What  is  said  in  the  note  with  regard  to  these  planets? 


280 


NATURAL  PHILOSOPHY. 


327.  Jupiter  is  the  largest  planet  of  the  solar  system, 
and  the  most  briUiant,  except  Venus.  The  heat  and 
light  at  Jupiter  is  about  25  times  less  than  that  at  the 
earth.  This  planet  is  attended  by  four  moons,  or  satel- 
lites ;  the  shadows  of  some  of  which  are  occasionally 
visible  upon  his  surface. 

1.  The  distance  of  those  satellites  from  the  planet  are  two, 
four,  six,  and  twelve  hundred  thousand  miles,  nearly. 

The  nearest  revolves  around  the  planet  in  less  than  two 
days ;  the  next  in  less  than  four  days ;  the  third  in  less  than 
eight  days ;  and  the  fourth  in  about  sixteen  days. 

2.  These  four  moons  must  afford  considerable  light  to  the 
inhabitants  of  the  planet;  for  the  nearest  appears  to  them 
four  times  the  size  of  our  moon :  the  second  about  the  same 
size  ;  the  third  somewhat  less  ;  and  the  fourth  about  one-third 
the  diameter  of  our  moon. 

3.  As  the  axis  of  Jupiter  is  nearly  perpendicular  to  its 
orbit,  it  has  no  sensible  change  of  seasons. 

4.  The  satellites  of  Jupiter  often  pass  behind  the  body  of 
the  planet,  and  also  into  its  shadow,  and  are  eclipsed.  These 
eclipses  are  of  use  in  ascertaining  the  longitude  of  places  on 
the  earth.  By  these  eclipses  also,  it  has  been  ascertained  that 
light  is  about  eight  minutes  in  coming  from  the  sun  to  the 
earth.  For,  an  eclipse  of  one  of  these  satellites  appears  to 
us  to  take  place  sixteen  minutes  sooner,  when  the  earth  is  in 
that  part  of  its  orbit  nearest  Jupiter,  than  when  in  the  part 
farthest  from  that  planet.  Hence,  light  is  sixteen  minutes  in 
crossing  the  earth's  orbit,  and  of  course  half  of  that  time,  or 
eight  minutes,  in  coming  from  the  sun  to  the  earth. 

sibility  by  Dr.  Brewster,  in  the  Edinburgh  Encyclopedia,  Art.  Astronomy. 
Dr.  Brewster  further  supposes,  that  the  bursting  of  this  planet  may  have 
occasioned  the  phenomena  of  meteoric  stones ;  that  is,  stones  which  have 
fallen  on  the  earth  from  the  atmosphere 


327.  Which  of  the  planets  is  the  largest?  How  much  more  light  and 
heat  does  the  earth  enjoy  than  Jupiter?  How  many  moons  has  this 
planet?  What  is  the  distance  of  these  moons  from  the  planet?  In  what 
time  do  they  perform  their  revolutions  around  the  planet  ?  How  does  the 
size  of  these  moons  compare  with  that  of  ours  ?  Why  has  Jupiter  no  sen- 
sible variety  of  seasons?  Of  what  use  are  the  eclipses  of  Jupiter's  moons  ? 
How  long  is  light  in  coming  from  the  sun  to  the  earth  How  has  this 
neen  ascertained^ 


ASTRONOMY. 


281 


5.  When  viewed  through  a  telescope,  several  belts  or  bands 
are  distinctly  seen,  sometimes  extending  across  his  disk,  and 
sometimes  interrupted  and  broken.  They  differ  in  distance, 
position,  and  number.  Tliey  are  generally  dark;  but  white 
ones  have  been  seen. 

G.  On  account  of  the  immense  distance  of  Jupiter  from  the 
sini,  and  also  from  Mercury,  Venus,  the  Earth,  and  Mars,  ob- 
servers on  Jupiter,  with  eyes  like  ours,  can  never  see  either  of 
the  above-named  planets,  because  they  would  always  be  im- 
mersed in  the  sun's  rays. 

828.  Saturn  is  the  second  in  size  and  the  last  but  one 
in  distance  from  the  sun.  The  degree  of  heat  and  light 
at  this  planet  is  eighty  times  less  than  that  at  the  earth. 

1.  Saturn  is  distinguished  from  the  other  planets  by  being 
encompassed  by  two  large,  luminous  rings.  They  reflect  the 
sun's  light  in  the  same  manner  as  his  moons.  They  are  en- 
tirely detached  from  each  other  and  from  the  body  of  the 
planet.  They  turn  on  the  same  axis  with  the  planet,  and  in 
nearly  the  same  time."^  The  edge  of  these  rings  is  constantly 
at  right  angles  with  the  axis  of  the  planet.  Stars  are  some- 
times seen  between  the  rings,  and  also  between  the  inner  ring 
and  the  body  of  the  planet.  The  breadth  of  the  two  rings  is 
about  the  same  as  their  distance  from  the  planet,  namety, 
21,000  miles.  As  they  cast  shadows  on  the  planet,  Dr.  Her- 
schel  thinks  them  sohd. 

2.  The  surface  of  Saturn  is  sometimes  diversified,  like  that 
of  Jupiter,  with  spots  and  belts.  Saturn  has  seven  satellites, 
or  moons,  revolving  around  him  at  different  distances,  and  in 
various  times,  from  less  than  one  to  eighty  days. 

3.  Saturn  may  be  known  by  his  pale  and  steady  light.  The 

*  These  rings  move  together  around  the  planet,  but  are  about  thirteen 
minutes  longer  in  performing  their  revolution  about  him,  than  Saturn  is  in 
rovolving  about  his  axis. 


How  does  Jupiter  appear  when  viewed  through  a  telescope  ? 

328.  How  does  Saturn  compare  in  size  with  the  other  planets  ?  How 
is  Saturn  distinguished  from  the  other  planets?  What  is  said  of  these 
rings  ?  How  much  longer  are  these  rings  in  performing  their  revolution 
around  the  planet  than  the  planet  is  in  performing  its  revolution  on  its  axis? 
What  m  the  breadth  of  these  rings?  What  is  said  of  the  surface  of  Sat- 
urn ?    How  many  moons  has  Saturn  ?    How  may  Saturn  be  known  I 


282 


NATURAL  PHILOSOPHY. 


seven  moons  of  Saturn,  except  one,  revolve  at  different  dis- 
tances  around  the  outer  edge  of  his  rings.  Dr.  Herschel  saw 
them  moving  along  it,  hke  bright  beads  on  a  white  string. 
They  do  not  often  suffer  eclipse  by  passing  into  the  shadow  of 
the  planet,  because  the  ring  is  generally  in  an  oblique  direc- 
tion. 

329.  Uranus,*  the  third  in  size,  is  the  most  remote  of 
all  the  old  planets.  It  is  scarcely  visible  to  the  naked  eye. 
The  light  and  heat  at  Uranus  are  about  360  times  less 
than  that  at  the  earth. 

1.  This  planet  was  formerly  considered  a  small  star;  but 
Dr.  Herschel,  in  1781,  discovered,  from  its  motion,  that  it  is  a 
planet. 

2.  Uranus  is  attended  by  six  moons,  or  satellites,  all  of 
which  were  discovered  by  Dr.  Herschel ;  and  all  of  them  re- 
volve in  orbits  nearly  perpendicular  to  that  of  the  planet. 
Their  motion  is  apparently  retrograde ;  but  this  is  probably  an 
optical  illusion,  arising  from  the  difficulty  of  ascertaining  which 
part  of  their  orbit  inchnes  towards  the  earth,  and  which  de- 
clines from  it.f 

*  This  planet  was  long  known  by  the  name  of  Herschel,  the  discoverer^ 
who,  in  announcing  his  discovery,  named  it  the  Georgium  sidus,"  in 
honor  of  King  George  III.  The  name  of  Uranus  was  given  to  it  by  the 
continental  astronomers. 

t  It  appears  to  be  a  general  law  of  satellites,  or  moons,  that  they  turn 
on  their  axes  in  the  same  time  in  which  they  revolve  around  their  pri- 
maries. On  this  account,  the  inhabitants  of  secondary  planets  observe 
6ome  singular  appearances,  which  the  inhabitants  of  primar)-  planets  do 
not.  Those  who  dwell  on  the  side  of  a  secoudar)'  planet  next  to  the 
primar}^,  will  always  see  that  primary ;  while  those  who  live  on  the  op- 
posite side  will  never  see  it.  Those  who  always  see  the  primar}^,  will  see 
it  constantly  in  very  nearly  the  same  place.    For  example,  those  who 


What  is  said  of  the  moons  of  Saturn  ?  Why  are  they  not  often  eclipsed  ? 

329.  How  does  Uranus  compare  in  size  with  the  other  planets?  How 
does  the  light  and  heat  at  Uranus  compare  with  that  of  the  earth  ?  By 
whom  was  this  planet  discovered?  What  name  did  he  give  it?  How 
many  moons  has  Uranus  ?  By  whom  were  they  discovered  ?  How  are 
their  orbits  situated,  with  regard  to  that  of  the  planet?  What  is  said  of 
their  motion?  Note.  What  appears  to  be  a  general  law  of  satellites? 
What  follows  from  this  with  regard  to  the  appearances  which  the  inhabi- 
tdnts  of  the  secondary  planets  must  observe  ? 


ASTRONOMY. 


283 


3.  It  is  a  singular  circumstance,  tliat,  before  the  discovery 
of  llerschel,  some  disturbances  and  deviations  were  observed 
by  astronomers  in  tlie  motions  of  Jupiter  and  Saturn,  whicli 
tlu^y  could  account  for  only  on  the  supposition  that  these  two 
planets  were  influenced  by  the  attraction  of  some  mor^  remote 
and  undiscovered  planet.  The  discovery  of  Herschel  com- 
pletely verified  their  opinions,  and  shows  the  extreme  nicety 
with  which  astronomers  observe  the  motions  of  planets. 

330.  The  planet  Neptune  is  a  recent  discovery  of 
Verrier.    But  little  is  as  yet  known  with  regard  to  it. 

331.  The  word  comet  is  derived  from  a  Greek  word, 
which  means  hair;  and  this  name  is  given  to  a  numerous 
class  of  bodies,  which  occasionally  visit,  and  appear  to 
belong  to  the  solar  system.  These  bodies  seem  to  con- 
sist of  a  nucleus,  attended  with  a  lucid  haze,  sometimes 
resembling  flowing  hair ;  from  whence  the  name  is  de- 
rived. Some  comets  appear  to  consist  wholly  of  this 
hazy  or  hairy  appearance,  which  is  frequently  called  the 
tail  of  the  comet.* 

dwell  near  the  edge  of  the  moon's  disk,  will  always  see  the  earth  near  the 
horizon,  and  those  in  or  near  the  centre  will  always  see  it  directly  or 
nearly  overhead.  Those  who  dwell  in  the  moon's  south  hmb  will  see  the 
earth  to  the  northward  ;  those  in  the'  north  limb  will  see  it  to  the  south- 
ward ;  those  in  the  east  limb  will  see  it  to  the  westward ;  while  those  in 
the  west  limb  will  see  it  to  the  eastward  ;  and  all  will  see  it  nearer  the  ho- 
rizon in  proportion  to  their  own  distance  from  the  centre  of  the  moon's 
disk.  Similar  appearances  are  exhibited  to  the  inhabitants  of  all  second- 
ary planets.  These  observations  are  predicated  on  the  suppositibn  that 
the  moon  is  inhabited.  But  it  is  not  generally  behoved  that  our  moon  is 
inhabited,  or  in  its  present  condition  fitted  for  the  residence  of  any  class 
of  beings. 

*  In  ancient  times,  the  appearance  of  comets  was  regarded  with  super- 
stitious fear,  in  the  belief  that  they  were  the  forerunners  of  some  direful 
calamity.  These  fears  have  now  been  banished,  and  the  comet  is  viewed 
as  a  constituent  member  of  the  system,  governed  by  the  same  harmonious 
and  unchanging  laws  which  regulate  and  control  all  the  other  heavenly 
bodies. 

The  number  of  comets  that  have  occasionally  appeared  within  the 

330.  Who  discovered  the  planet  Neptune  ? 

331.  What  is  the  meaning  of  the  word  comet?  To  what  class  of  bodies 
is  this  name  given?    Of  what  do  these  bodies  appear  to  consist? 


284  NATURAL  PHILOSOPHY. 

1  Comets,  in  their  revolution,  describe  long  narrow  ovals. 
They  approach  very  near  the  sun  in  one  of  the  ends  of  these 
ovals ;  and  when  they  are  in  the  opposite  end  of  the  orbit, 
their  distance  from  the  sun  is  immensely  great. 

2  The  extreme  nearness  of  approach  to  the  sun  gives  to  a 
comet,  when  in  perihelion,  a  swiftness  of  motion  prodigiously 
ffi-eat.  Newton  calculated  the  velocity  of  the  comet  ot  1680 
to  be  880,000  miles  an  hour.  This  comet  was  remarkable  tor 
its  near  approach  to  the  sun,  being  no  further  than  580,000 
miles  from  it,  which  is  but  little  more  than  half  the  sun  s 
diameter.  Brydone  calculated  that  the  velocity  of  a  comet, 
which  he  observed  at  Palermo,  in  1770,  was  at  the  rate  oi 
two  millions  and  a  half  of  miles  in  an  hour. 

3  The  luminous  stream,  or  tail,  of  a  comet,  follows  it  as  it 
approaches  the  sun,  and  goes  before  it  when  the  comet  recedes 
from  the  sun.  Newton,  and  some  other  astronomers,  consider- 
ed the  tails  of  comets  to  be  vapors,  produced  by  the  excessu-;e 
heat  of  the  sun.  Of  whatever  substance  they  may  be,  it  is 
certain  that  it  is  very  rare,  because  the  stars  may  be  distinctly 
seen  through  it. 

limits  of  the  solar  system  is  variously  stated,  from  350  to  500.  The  paths 
or  orbits  of  about  98  of  these  have  been  calculated  from  observation  of  the 
times  at  which  they  most  nearly  approached  the  sun ;  their  distance  from 
it,  and  from  the  earth,  at  those  times  ;  the  direction  of  their  movements, 
whether  from  east  to  west,  or  from  west  to  east;  and  the  places  m  the 
starry  sphere  at  which  their  orbits  crossed  that  of  the  earth,  and  their  in- 
clination to  it.  The  result  is,  that,  of  these  98,  24  passed  hetween  the  sun 
and  Mercury,  33  passed  between  Mercury  and  Venus,  21  between  Venus 
and  the  earth,  16  between  the  earth  and  Mars,  3  between  Mars  and  Ceres, 
and  1  between  Ceres  and  Jupiter;  that  50  of  these  comets  moved  from 
east  to  west ;  that  their  orbits  were  inclined  at  every  possible  angle  to  that 
of  the  earth.  The  greater  part  of  them  ascended  above  the  orbit  of  the 
earth,  when  very  near  the  sun ;  and  some  were  observed  to  dash  down 
from  the  upper  regions  of  space,  and,  after  turning  round  the  sun,  to  mount 

again.   

What  is  the  number  of  comets  that  have  occasionally  appeared  ?  What 
discoveries  have  been  made  concerning  98  of  them  ?  What  is  the  result  ? 
What  is  the  form  of  the  orbits  of  comets?  What  is  said  of  the  motion  of 
comets  when  in  perihelion  ?  What  did  Newton  calculate  the  velocity  of 
the  comet  of  1680  to  be  in  an  hour?  For  what  was  this  comet  remarka- 
ble ^  What  is  said  of  the  luminous  stream  of  a  comet  as  it  approaches 
and  recedes  from  the  sun?  What  did  Newton,  and  some  other  astrono- 
mers,  consider  the  tails  of  comets  to  be  ? 


ASTKO^  ')]M  V. 


285 


4.  Tlie  tails  of  comets  differ  very  gi'catly  in  length,  and 
some  are  attended  apparently  by  only  a  small  cloudy  light, 
while  the  length  of  the  tail  of  others  has  been  estimated  at 
from  50  to  80  millions  of  miles.* 

*  It  has  been  argued  that  comets  consist  of  very  little  solid  substance, 
because,  although  they  sometimes  approach  very  near  to  the  other  heaven- 
ly bodies,  they  appear  to  exert  no  sensible  attractive  force  upon  those 
bodies.  It  is  said  that,  in  1454,  the  moon  was  eclipsed  by  a  comet.  The 
comet  must,  therefore,  have  been  very  near  the  earth,  (less  than  240,000 
miles ;)  yet  it  produced  no  sensible  effect  on  the  earth  or  the  moon ;  for 
it  did  not  cause  them  to  make  any  perceptible  deviation  from  their  ac- 
customed paths  round  the  sun.  It  has  been  ascertained  that  comets  are 
disturbed  by  the  gravitating  power  of  the  planets ;  but  it  does  not  appear 
that  the  planets  are  in  like  manner  affected  by  comets. 

Many  comets  escape  observation,  because  they  traverse  that  part  of 
the  heavens  only  which  is  above  the  horizon  in  the  daytime.  They  are, 
therefore,  lost  in  the  brilliancy  of  the  sun,  and  can  be  seen  only  when  a 
total  eclipse  of  the  sun  takes  place.  Seneca,  60  years  before  the  Chris- 
tian  era,  states  that  a  large  comet  was  actually  observed  very  near  the 
sun,  during  an  eclipse. 

Dr.  Halley  and  Professor  Encke  and  Biela  are  the  first  astronomers  that 
ever  successfully  predicted  the  return  of  a  comet.  The  periodical  time  of 
Halley's  comet  is  about  76  years.  It  appeared  last  in  the  fall  of  1835 : 
that  of  Encke  is  about  1200  days ;  that  of  Biela  about  6|  years.  This 
last  comet  appeared  in  1832  and  in  1838. 

The  comet  of  1758,  the  return  of  which  was  predicted  by  Dr.  Halley, 
was  regarded  with  great  interest  by  astronomers,  because  its  return  was 
predicted.  But  four  revolutions  before,  in  1456,  it  was  looked  upon  with 
the  utmost  horror.  Its  long  tail  spread  consternation  over  all  Europe,  al- 
ready terrified  by  the  rapid  success  of  the  Turkish  arms.  Pope  Callixtus, 
on  this  occasion,  ordered  a  prayer,  in  which  both  the  comet  and  the  Turks 
were  included  in  one  anathema.  Scarcely  a  year  or  a  month  now  elap- 
ses without  the  appearance  of  a  comet  in  our  system.  But  it  is  now 
known  that  they  are  bodies  of  such  extreme  rarity,  that  our  clouds  are 
massive  in  comparison  with  them.  They  have  no  more  density  than  the 
air  under  an  exhausted  receiver.  Herschel  saw  stars  of  the  6th  magnitude 
through  a  thickness  of  30,000  miles  of  cometic  matter  The  number  of 
comets  in  existence  within  the  compass  of  the  solar  system,  is  stated  by 
some  astronomers  as  over  seven  millions  ! 


What  is  said  in  the  note  with  regard  to  comets?  Who  were  the  first 
astronomers  that  successfully  predicted  the  return  of  a  comet  ?  What  is 
the  periodical  time  of  Halley's  comet  ?    Of  Encke's  ?    Of  Biela's  ? 


286 


NATURAL  PHILOSOPHY. 


332.  The  stars  are  classed  into  six  magnitudes  :  the 
largest  are  of  the  first  magnitude,  and  the  smallest  that 
can  be  seen  by  the  naked  eye,  are  of  the  sixth.  Those 
stars  which  can  be  seen  only  by  means  of  telescopes, 
are  called  telescopic  stars. 

1.  The  distance  of  the  fixed  stars  cannot  be  determined,  be- 
cause we  have  no  means  of  ascertaining  the  distance  of  any 
body  which  exceeds  200  thousand  times  that  of  the  earth  from 
the  sun.  As  none  of  the  stars  comes  within  that  limit,  we  can- 
not determine  their  real  distance.  It  is  generally  supposed 
that  part,  if  not  all,  of  the  difference  in  their  apparent  magni- 
tudes is  owing  to  the  diff'erence  in  their  distance."^ 

2.  Although  the  stars  generally  appear  fixed,  they  all  have 
motion ;  but  their  distance  being  so  immensely  great,  a  rapid 
motion  would  not  perceptibly  change  their  relative  situation  in 
two  or  three  thousand  years.  Some  have  been  noticed  alter- 
natelv  to  appear  and  disappear :  several  that  were  mentioned 
by  ancient  astronomers,  are  not  now  to  be  seen ;  and  some  are 
now  observed,  which  were  unknown  to  the  ancients. 

3.  Many  stars  which  appear  single  to  the  naked  eye,  when 
viewed  through  powerful  telescopes,  appear  double,  treble,  and 
even  quadruple.  Some  are  subject  to  variation  in  their  ap- 
parent magnitude ;  at  one  time  being  of  the  second,  or  thu'd, 
and,  at  another,  of  the  fifth  or  sixth  magnitude. 

333.  The  Galaxy,  or  Milky  Way,  is  a  remarkably 
light  broad  zone,  visible  in  the  heavens,  passing  from 
northeast  to  southwest.  It  is  supposed  to  consist  of 
an  immense  number  of  stars,  which,  from  their  apparent 
nearness,  cannot  be  distinguished  from  each  other. 

1.  Dr.  Herschel  saw,  in  the  course  of  a  quarter  of  an  hour, 
the  astonishing  number  of  116,000  stars  pass  through  the  field 
of  his  telescope,  while  it  was  directed  to  the  milky  way. 

*  The  distance  of  the  stars,  according  to  Sir  J.  Herschel,  cannot  be 
less  than  19,200,000,000,000  niiles.  How  much  greater  it  really  is  we 
know  not. 

332.  Into  how  many  magnitudes  are  the  stars  classed  ?  Of  what  mag- 
nitude are  the  largest?  Of  what  are  the  smallest?  What  are  telescopic 
stars?  Why  cannot  the  distance  of  the  fixed  stars  be  determined  ?  To 
what  is  the  difference  in  their  apparent  magnitudes  supposed  to  be  owing? 
Have  the  stars  any  motion  ? 

333.  What  is  the  Galaxy  ?    Of  what  is  it  supposed  to  consist  ? 


Ai^TRONOMY. 


287 


2.  The  ancients,  in  reducing  astronomy  to  a  science,  formed 
tlie  stars  into  dusters,  or  constellations,  to  which  they  gave 
particular  names. 

3.  The  number  of  constellations  among  the  ancients  was 
about  fifty.    The  moderns  have  added  about  fifty  more.^ 

4.  Oil  a  celestial  globe,  the  largest  star  in  each  constellation 
is  usually  designated  by  the  first  letter  of  the  Greek  alphabet ; 
and  the  next  largest  by  the  second,  &c.  When  the  Greek 
alphabet  is  exhausted,  the  English  alphabet,  and  then  numbers, 
are  used. 

5.  The  stars,  and  other  heavenly  bodies,  are  never  seen  in 
then-  true  situation,  because  the  motion  of  light  is  progressive; 
and,  during  the  time  that  light  is  coming  to  the  earth,  the 
earth  is  constantly  in  motion.  In  order,  therefore,  to  see  a 
star,  the  telescope  must  be  turned  somewhat  before  the  star,  and 
in  the  direction  in  which  the  earth  moves. 

Hence,  a  ray  of  light  passing  through  the  centre  of  the 
telescope,  to  the  observer's  eye,  does  not  coincide  with  a  direct 
Ime  from  his  eye  to  the  star,  but  makes  an  angle  with  it ;  and 
this  IS  termed  the  aberration  of  light.\ 

6.  The  daily  rotation  of  the  earth  on  its  axis  causes  the 
whole  sphere  of  the  fixed  stars,  &c.,  to  appear  to  move  round 
the  earth  every  twenty-four  hours  from  east  to  west.  To  the 
inhabitants  of  the  northern  hemisphere,  the  immoveable  point, 

*  Our  observations  of  the  stars  and  nebulae  are  confined  principally  to 
those  of  the  northern  hemisphere.  Of  the  constellations  near  the  south 
pole  we  know  but  little. 

t  In  determining  the  true  place  of  any  of  the  celestial  bodies,  the  re- 
fractive power  of  the  atmosphere  must  always  be  taken  into  consideration. 
This  property  of  the  atmosphere  adds  to  the  length  of  the  days,  by  causing 
the  sun  to  appear  before  it  has  actually  risen,  and  by  detaining  its  appear- 
.  ance  after  it  has  actually  set. 

How  did  the  ancients  divide  the  stars?  What  was  the  number  of  con- 
stellations among  the  ancients  ?  How  many  have  been  added  by  the  mod- 
erns ?  How  are  the  stars  designated  on  the  celestial  globe  ?  What  is  the 
situation  of  each  constellation  now  ?  Illustrate  this.  What  is  the  cause 
of  this  difference  ?  Why  do  we  not  see  the  stars,  and  other  heavenly 
bodies,  in  their  true  situation?  How  can  a  star  be  seen  in  its  true  situa- 
tion ?  What  is  meant  by  the  aberration  of  light  ?  What  is  necessary  to 
be  taken  into  consideration,  in  determining  the  true  place  of  the  celestial 
bodies  ?  What  effect  has  this  property  of  the  atmosphere  on  the  length 
of  the  days  ?  ^ 


288 


NATURAL  PHILOSOPHY. 


on  which  the  whole  seems  to  turn,  is  the  Pole  Star.  To  the 
inhabitants  of  the  southern  hemisphere,  there  is  another,  and 
a  corresponding  point  in  the  heavens. 

7.  Certain  of  the  stars  surrounding  the  north  pole,  never 
set  to  us.  These  are  included  in  a  circle  parallel  with  the 
equator,  and  in  every  part  equally  distant  from  the  north  pole 
star.  This  circle  is  called  the  circle  of  perpetual  apparition. 
Others  never  rise  to  us ;  these  are  included  in  a  circle  equally 
distant  from  the  south  pole ;  and  this  is  called  the  circle  of 
perpetual  occultation.  Some  of  the  constellations  of  the  south- 
ern hemisphere  are  represented  as  inimitably  beautiful,  par- 
ticularly the  cross. 

334.  The  parallax  of  a  heavenly  body  is  the  difFerence 
between  the  true  and  the  apparent  situation  of  the  body. 

1.  In  Fig.  158,  AGB  represents  the  earth,  and  C  the 
moon.  To  a  spectator  at  A,  the  moon  would  appear  at  F  ; 
while  to  another  at  B,  the  moon  would  appear  at  D  ;  but  to  a 


F 


third  spectator  at  G,  the  centre  of  the  earth,  the  moon  would 
appear  at  E,  which  is  the  true  situation.  The  distance  from  F 
to  E  is  the  parallax  of  the  moon  when  viewed  from  A,  and  the 
distance  from  E  to  D  is  the  parallax  when  viewed  from  B. 

2.  From  this  it  appears,  that  the  situation  of  the  heavenly 
bodies  must  always  be  calculated  from  the  centre  of  the  earth ; 
and  the  observer  must  always  know  the  distance  between  the 
place  of  his  observation  and  the  centre  of  the  earth,  in  order  to 
make  the  necessary  calculations,  to  determine  the  true  situa- 
tion of  the  body.  Allowance  also  must  be  made  for  the  re- 
fraction of  the  atmosphere. 


334.  What  is  the  parallax  of  a  heavenly  body?  Explain  Fig.  158. 
What  appears  from  this  ?    What  allowance  must  also  be  made  ? 


ASTRONOMY. 


289 


335.  The  moon  is  a  secondary  planet,  revolving  about 
the  earth  in  about  29|  days.  Its  distance  from  the 
earth  is  about  240,000  miles.  It  turns  on  its  axis  in 
precisely  the  same  time  that  it  performs  its  revolution 
about  the  earth.  Consequently  it  always  presents  the 
same  side  to  the  earth. 

1.  The  most  obvious  fact  in  relation  to  the  moon,*  is  that 

*  The  surface  of  the  moon  appears  to  be  volcanic  and  very  mountain- 
ous. Occasional  volcanoes  have  been  seen  in  action  on  the  dark  side. 
No  heat  has  been  detected  in  the  moon's  rays,  even  when  most  powerfully 
concentrated,  that  will  affect  the  most  delicate  thermometer ;  and  hence 
some  have  inferred  that  the  moon  extracts  the  heat  from  the  smi's  rays 
before  it  reflects  them. 

One  of  the  most  common  errors,  with  regard  to  the  moon,  is  that  which 
ascribes  to  it  an  influence  over  the  weather.  Tables  of  the  weather  have 
been  compared  with  the  lunar  phases  for  a  period  of  a  hundred  years,  and 
over  a  thousand  lunations,  during  which  time  about  491  new  or  full  moons 
have  been  attended  by  a  change  of  the  weather,  and  509  have  not. 

The  moon  is  equally  innocent  of  putrefaction,  notwithstanding  the  pop 
ular  belief  that  it  hastens  that  process,  especially  in  fish.  The  same  cause 
which  produces  dew,  causes  moisture  on  substances  exposed  to  it,  and  this 
moisture  is  the  real  cause  of  putrefaction. 

Dr.  Olbers  of  Bremen,  by  a  comparison  of  a  great  number  of  cases,  ar- 
rived at  the  conclusion  that  the  moon  has  no  effect  on  insanity ;  although 
the  popular  belief  is  that  the  fits  are  aggravated  or  affected  by  the  lunar 
phases. 

The  density  of  the  moon  is  estimated  at  about  one-fifth  that  of  the 
earth  ;  hence  10  pounds  on  the  earth  will  be  equal  to  2  on  the  moon. 
The  days  and  nights  on  the  moon  are  each  equal  to  14  of  our  days.  The 
axis  of  the  moon  is  p'-rpendicular  to  its  orbit,  and  therefore  the  moon  can 
have  no  variety  of  seasons.  The  moon  likewise  has  no  atmosphere,  and 
therefore  it  cannot  be  inhabited  ;  for  there  can  be  no  vegetation,  no  clouds, 
no  ocean,  no  liquids,  no  light  in  dwellings,  no  twilight ;  in  short,  nothing 
that  could  fit  it  for  the  habitation  of  any  order  of  beings  with  which  we 
are  acquainted. 

In  connexion  with  what  has  now  been  stated  with  regard  to  the  moon 
and  its  volcanic  appearances,  it  will  be  proper  to  notice  the  subject  of  aero- 
iites,  or  meteoric  stones  ;  because,  accordinor  to  the  opinion  of  some,  they 
are  of  lunar  origin.    Three  theories  have  been  broached  with  regard  to 

335.  Is  the  moon  a  primary  or  secondary  planet?  How  long  is  it  in 
performing  its  revolution  about  the  earth  ?  What  is  its  distance  from  the 
earth  ? 

13 


290  NATURAL  PHILOSOPHY. 

its  disk  is  constantly  changing  its  appearance  sometimes  only 
a  semicircular  edge  being  illuminated,  while  the  rest  is  dark ; 
at  another  time,  the  whole  surface  appearing  resplendent. 
This  is  caused  by  the  relative  position  of  the  moon  with 
regard  to  the  sun  and  the  earth.    The  moon  is  an  opaque 
body,  and  shines  only  by  the  hght  of  the  sun.    When,  there- 
fore, the  moon  is  between  the  earth  and  the  sun,  it  presents 
its  dark  side  to  the  earth;  while  the  side  presented  to  the 
sun,  and  on  which  the  sun  shines,  is  invisible  to  the  eartli. 
But  when  the  earth  is  between  the  sun  and  the  moon,  the  illu- 
minated side  of  the  moon  is  visible  at  the  earth       ,  ,  _  ^  „ 
2   In  Fiff.  159,  let  S  be  the  sun,  E  the  earth,  and  A  B  O  V 
the  moon  in  diflerent  parts  of  her  orbit.   When  the  moon  is  at 
A  its  dark  side  will  be  towards  the  earth,  its  illuminated  part 
being  always  towards  the  sun.    Hence  the  moon  will  appear 
to  us  as  represented  at  a.    But  when  it  has  advanced  m  its 
orbit,  to  B,  a  small  part  of  its  illuminated  side  commg  in  sight,  it 
appears  as  represented  at  b,  and  is  said  to  be  horned  When 
it  arrives  at  C,  one-half  its  illuminated  side  is  visible,  and  it 
appears  as  at  c.    At  C,  and  in  the  opposite  point  of  its  orbit, 
the  moon  is  said  to  be  in  quadrature.    At  D  its  appearance 

them:  1st,  that  they  are  formed  in  the  air,  from  materials  existing  there 
in  a  sublimated  state  ;  2d,  that  they  are  parts  of  an  exploded  planet ;  3d, 
that  they  are  thrown  from  the  volcanoes  in  the  moon.  .         „  , 

To  the  first  of  these  theories  there  is  a  material  objection,  m  the  tact 
that  gases,  when  in  contact,  must  mix,  and  gases  necessary  to  form  these 
substances  cannot,  therefore,  remain  in  the  air  unmixed. 

To  the  second  hypothesis  it  may  be  objected,  that  if  they  were  parts  of 
a  broken  planet,  they  would  probably  be  composed  of  more  heterogeneous 
materials.  But  it  is  well  known  that  all  of  them  are  composed  of  the  same 
constituent  parts,  namely ;  sulphur,  magnesia,  manganese,  iron,  nickel, 
chromium,  and  in  one  recorded  instance  only,  charcoal. 

In  favor  of  the  third  supposition,  which  refers  them  to  a  lunar  ori- 
rin  it  may  be  remarked,  that  a  body  thrown  70  miles  from  the  moon, 
would  escape  from  the  moon's  attraction;  and  that  a  velocity  six  times 
greater  than  that  of  a  cannon-ball,  would  be  sufficient  to  throw  a  body 
beyond  the  moon's  attraction.  As  terrestrial  volcanoes  have  thrown  bodies 
with  this  velocity,  it  is  not  improbable  that  lunar  volcanoes  may  do  the 


same. 


What  is  the  most  obvious  fact  in  relation  to  the  moon  ?  How  is  this 
caused?  What  kind  of  a  body  is  the  moon?  By  what  light  does  it 
shine  ? 


ASTRONOMY. 


291 


is  as  represented  at  ^/,  and  it  is  said  to  be  gibhoui^.  At  E  all 
illuminated  side  is  towards  us,  and  we  have  a  full  moon. 
During'  the  other  half  of  its  revolution,  less  and  less  of  its 
illuminated  side  is  seen,  till  it  again  becomes  invisible  at  A.* 

Fig.  159. 


c 


3.  The  mean  difference  in  the  rising  of  the  moon,  caused  by 
its  daily  motion,  is  a  little  less  than  an  hour.  But  on  account 
of  the  different  angles  formed  with  the  horizon  by  different 
parts  of  the  ecliptic,  it  happens  that  for  six  or  eight  nights 

*  The  following  signs  are  used  in  our  common  almanacs  to  denote  the 
different  positions  and  phases  of  the  moon.  )  or  ])  denotes  the  moon  in 
the  first  quadratm-e  ;  that  is,  the  quadrature  between  change  and  full ; 
(  or  (I  denotes  the  moon  in  the  last  quadrature ;  that  is,  the  quadrature 
between  full  and  change ;  Q  denotes  new  moon  ;  %  denotes  full  moon. 

When  viewed  through  a  telescope,  the  surface  of  the  moon  appears 
wonderfully  diversified.  Large  dark  spots,  supposed  to  be  excavations  or 
valleys,  are  visible  to  the  eye  ;  some  parts  also  appear  more  lucid  than  the 
general  surface.  These  are  ascertained  to  be  mountains,  by  the  shadows 
which  they  cast.  Maps  of  the  moon's  surface  have  been  drawn,  on 
which  most  of  these  valleys  and  mountains  are  delineated,  and  names  are 
given  to  them.  Some  of  these  excavations  are  thought  to  be  4  miles  deep 
and  40  wide.  A  high  ridge  generally  surrounds  them,  and  often  a  moun- 
tain rises  in  the  centre.  These  immense  depressions  probably  very  much 
resemble  what  would  be  the  appearance  of  the  earth  at  the  moon,  were  all 
the  seas  and  lakes  dried  up.  Some  of  the  mountains  are  supposed  to  be 
volcanic. 


How  does  the  moon  appear  when  viewed  through  a  telescope  ?  What 
causes  the  difference  in  the  rising  of  the  moon  ?  What  is  the  mean  differ- 
ence in  the  rising  of  the  moon  ? 


292  NATURAL  PHILOSOPHY. 

near  tlie  full  moons  of  September  and  October,  the  moon  rises 
nearly  as  soon  as  the  sun  is  set.  As  this  is  a  great  convenience 
to  the  husbandman  and  the  hunter,  inasmuch  as  it  affords 
them  lio-ht  to  continue  their  occupation,  and,  as  it  were,  length- 
ens out  their  day,  the  first  is  called  the  harvest  moon,  and 
Ihe  second  the  hunter's  moon.  These  moons  are  always 
most  beneficial  when  the  moon's  ascending  node  is  m  or  near 


ines: 


OF  THE  TIDES. 

336.  The  tides  are  the  regular  rising  and  falling  of 
the  water  of  the  ocean  twice  in  about  25  hours,  i  hey 
are  occasioned  by  the  attraction  of  the  moon  upon  the 
matter  of  the  earth  ;  and  they  are  also  affected  by  that 
of  the  sun. 

1  Let  M  Fio-.  160,  be  the  moon  revolving  in  her  orbit ;  E  the 
earth  covered  with  water,  and  S  the  sun.    Now  the  weight  of 


Fig.  160. 


*  The  reader  who  wishes  a  simple  and  clear  illustration  of  the  causes 
which  produce  the  harvest  moon,  is  referred  to  Wilkins's  Astronomy,  page 
M  To  hose  who  wish  a  fuller  treatise  on  astronomy  than  .s  contame^ 
in  this  volume,  Phillip's  "  Eight  Familiar  Lectures"  are  recommended 
They  give  a  clear  illuLtion  o'f  those  portions  of  the  subject  wh.ch  do  not 
require  the  aid  of  the  mathematics. 


When  are 


ASTRONOMY. 


293 


a  particle  of  matter  is  its  tendency  towards  the  centre  of  tho 
earth,  and  whatever  goes  to  separate  the  centre  from  any  par- 
ticle, dnninishes  that  tendency,  and  consequently  lessens  the 
weight  in^  the  same  proportion.  Now  since  the  water  at  and 
about  A,  is  nearer  the  moon  than  that  between  A  and  C,  and 
between  A  and  D,  it  will  be  more  strongly  attracted;  and 
consequently,  such  attraction  will  diminish  the  weight  of  the 
water  more  at  A  than  at  the  points  between  A  and  C  and 
between  A  and  D  :  hence,  the  water  being  made  lighter  at  A, 
will  by  the  force  of  the  hydrostatic  pressure  rise,  until  the 
whole  mass  comes  into  equilibrium. 

Again,  the  waters  at  C  and  D,  and  between  C  and  B  and  D 
and  B,  being  nearer  the  moon  than  the  water  at  B,  will  be 
more  attracted  by  it,  and  hence  its  weight  will  be  increased 
more  than  that  of  the  water  at  B,  which  latter  weight  is  also 
dimmished  by  the  effort  of  the  moon  to  separate  the  centre  E, 
and  the  particle  of  matter  at  B.  Therefore,  the  water  being 
made  lighter  at  B  will  rise,  the  same  as  at  A,  and  this  rise 
will  also  be  increased  by  the  motion  of  the  moon  and  earth 
around  their  common  centre  of  gravity. 

2.  Thus  any  particular  place,  as  A,  while  passing  from  under 
the  moon  till  it  comes  under  the  moon  again,  has  two  tides. 
But  the  moon  is  constantly  advancing  in  its  orbit,  so  that  the 
earth  must  a  little  more  than  complete  its  rotation,  before  the 
place  A  comes  under  the  moon.  This  causes  high  water 
at  any  place  about  fifty  minutes  later  each  successive  day. 

3.  As  the  moon's  orbit  varies  but  Httle  from  the  ecliptic 
the  moon  is  never  more  than  29^  from  the  equator,  and  is 
generally  much  less.  Hence  the  waters  about  the  equator 
being  nearer  the  moon,  are  more  strongly  attracted,  and  the 
tides  are  higher  than  towards  the  poles. 

4.  The  sun  attracts  the  waters  as  well  as  the  moon.  When 
the  moon  is  at  full  or  change,  being  in  the  same  line  of  direc- 
tion, (see  Fig.  160,)  the  sun  acts  with  it ;  that  is,  the  sun  and 
moon  tend  to  raise  the  tides  at  the  same  place,  as  seen  in  the 
figure.  The  tides  are  then  very  high,  and  are  called  spring 
tiQ.es. 

But  when  the  moon  is  in  its  quarters,  as  in  Fig.  161,  the 


Explain  the  theory  from  Fig.  160.  What  is  the  greatest  distance  of  the 
moon  from  the  equator?  Where  are  the  tides  highest?  Why?  Has 
the  sun  any  effect  on  the  tides  ?    What  are  spring  tides  ?    When  do  they 


294  NATURAL  PHILOSOPHY. 

sun  and  moon  being  in  lines  at  right  angles  tend  to  raise  tides 

Fig.  16L 


at  different  places  ;  namely,  the  moon  at  C  and  D,  and  the  sun 
at  A  and  B.  Tides  that  are  produced  when  the  moon  is  m  its 
quarters,  are  low,  and  are  called  neap  tides.'' 


OF  ECLIPSES. 


337.  An  eclipse  is  a  total  or  partial  obscuration  of  one 
heavenly  body  by  the  intervention  of  another. 

1  The  situation  of  the  earth  with  regard  to  the  moon,  or 
rather  of  the  moon  with  regard  to  the  earth,  occasions  eclipses 
both  of  the  sun  and  moon.  Those  of  the  sun  take  place  when 
the  moon,  passing  between  the  sun  and  earth,  intercepts  his 
rays  Those  of  the  moon  take  place  when  the  earth  coming 
between  the  sun  and  moon,  deprives  the  moon  of  his  hght. 
Hence,  an  eclipse  of  the  sun  can  take  place  only  when  the  moon 
changes,  and  an  eclipse  of  the  moon  only  when  the  moon  lulls ; 

'  There  are  so  manv  natural  difficulties  to  the  free  progress  of  the  tides, 
,a  the  theory-  by  which  they  are  accounted  for  is,  in  fact,  and  necessarily, 
the  most  imperfect  of  all  the  theories  connected  with  astronomy.  It  is 
however,  indisputable,  that  the  moon  has  an  effect  upon  the  tides,  although 
it  be  not  equally  felt  in  all  places,  owing  to  the  indentations  of  the  coast, 
the  obstructions  of  islands,  continents,  &c.,  which  prevent  the  free  motion 
of  the  waters.  In  narrow  rivers,  the  tides  are  frequently  very  high  and 
sudden,  from  the  resistance  afforded  by  their  banks  to  the  free  ingress  of 
the  water,  whence  what  would  otherwise  be  a  tide,  becomes  an  accamu- 
lation  It  has  been  constantly  observed,  that  the  spring  tides  happen  at 
the  new  and  full  moon,  and  the  neap  tides  at  the  quarters.  Th,s  circum- 
stance is  sufficient  in  itself  to  prove  the  connexion  between  the  influence 
of  the  moon  and  the  tides. 

What  are  neap  tides  ?    When  do  they  take  place  ? 
J37.  What  is  an  eclipse  ?   When  does  an  eclipse  of  the  sun  take  place  7 
Wbea  does  an  eclipse  of  the  moon  take  place  ? 


ASTRONOMY. 


295 


for  at  the  time  of  an  echp'^^e,  either  of  the  sun  or  tlic  moon,  the 
sun,  earthy  and  moon  must  he  in  the  same  straight  line. 

2.  If  the  moon  revolved  around  tlie  earth  in  the  same  plane 
in  which  the  earth  revolves  around  the  sun,  that  is,  in  the 
ecliptic,  it  is  plain  that  the  sun  would  be  eclipsed  at  every  new 
moon ;  and  the  moon  would  be  eclipsed  at  every  full.  For  at 
each  of  these  times,  these  three  bodies  would  be  in  the  same 
straight  line.  But  the  moon's  orbit  does  not  coincide  with  the 
ecliptic,  but  is  inclined  to  it  at  an  angle  of  about  5°  20^.  Hence, 
since  the  apparent  diameter  of  the  sun  is  but  about  i  a  degree,  and 
that  of  the  moon  about  the  same,  no  eclipse  will  take  place  at 
new  or  full  moon,  unless  the  moon  be  within  i  a  degree  of  the 
ecliptic,  that  is,  in  or  near  one  of  its  nodes.  It  is  found  that 
if  the  moon  be  within  16^^  of  a  node  at  time  of  change,  it  will 
be  so  near  the  ecliptic,  that  the  sun  will  be  more  or  less 
echpsed;  if  within  12°  at  time  of  full,  the  moon  will  be  more 
or  less  eclipsed. 

3.  It  is  obvious  that  the  moon  will  be  oftener  within  16^-^  of 
a  node  at  the  time  of  change,  than  within  12^  at  the  time  of 
full ;  consequently  there  will  be  more  eclipses  of  the  sun  than 
of  tue  moon  in  a  course  of  years.  As  the  nodes  commonly 
come  between  the  sun  and  earth  but  twice  in  a  year,  and  the 
moon's  orbit  contains  360O,  of  which  16^o,  the  limit  of  solar 
eclipses,  and  12°,  the  limit  of  lunar  eclipses,  are  but  small 
portions,  it  is  plain  there  must  be  many  new  and  full  moons 
without  any  eclipses. 

4.  Altnough  there  are  more  eclipses  of  the  sun  than  of  the 
moon,  yet  more  eclipses  of  the  moon  will  be  visible  at  a  par- 
ticular place,  as  Boston,  in  a  course  of  years,  than  of  the  sun. 
Since  the  sun  is  very  much  larger  than  either  the  earth  or 
moon,  the  shadow  of  these  bodies  must  always  terminate  in  a 
ponit;  that  is,  it  must  always  be  a  cone.  In  Fig.  162,  let  S 
be  the  sun,  m  the  moon,  and  E  the  earth.  The  sun  constantly 
illuminates  half  the  earth's  surface,  that  is,  a  hemisphere ;  and 
consequently  st  is  visible  to  all  in  this  hemisphere.    But  the 


What  is  necessary  at  the  time  of  an  eclipse  ?  How  often  would  there 
be  an  eclipse,  if  the  moon  went  round  the  earth  in  the  same  plane  in  which 
the  earth  goes  round  the  sun?  Why?  What  is  the  inchnation  of  the 
moon's  orbit  to  the  ecliptic  ?  What  is  the  apparent  diameter  of  the  sun 
and  moon  ?  What  follows  from  this  ?  When  is  the  sun  eclipsed  ?  When 
the  moon  ?  Does  an  eclipse  happen  every  time  there  is  a  full  or  new 
moon  ?    What  must  the  shadows  of  these  bodies  always  b©  ?    Why  ? 


296 


NATURAL  PHILOSOPHY. 


moon's  shadow  falls  upon  a  part  only  of  this  hemisphere ;  and 
hence  the  sun  appears  ecUpsed  to  a  part  only  of  those  to  whom 


Fig.  162. 


it  is  visible.  Sometimes,  when  the  moon  is  at  its  greatest  dis- 
tance, its  shadow,  0  m,  terminates  before  it  reaches  the  earth. 
Tn  eclipses  of  this  kind,  to  an  inhabitant  directly  under  the 
point  O,  the  outermost  edge  of  the  sun's  disk  is  seen,  forming 
a  bright  ring  around  the  moon ;  from  which  circumstance 
these  eclipses  are  called  annular,  from  annulus,  a  Latin  word 
for  ring. 

Besides  the  dark  shadow  of  the  moon,  m  0,  in  which  all  the 
light  of  the  sun  is  intercepted,  (in  which  case  the  eclipse  is 
called  total,)  there  is  another  shadow,  r  C  D  S,  distinct  from 
the  former,  which  is  called  the  'penumbra.  Within  this,  only  a 
part  of  the  sun's  rays  are  intercepted,  and  the  eclipse  is  called 
partial.  If  a  person  could  pass,  during  an  eclipse  of  the  sun, 
from  0  to  D,  immediately  on  emerging  from  the  dark  shadow, 
0  m,  he  would  see  a  small  part  of  the  sun ;  and  would  con- 
tinually see  more  and  more  till  he  arrived  at  D,  where  all 
shadow  would  cease,  and  the  whole  sun's  disk  be  visible.  Ap- 
pearances would  be  similar  if  he  went  from  0  to  C.  Hence 
the  penumbra  is  less  and  less  dark,  (because  a  less  portion  of 
the  sun  is  eclipsed,)  in  proportion  as  the  spectator  is  more  re- 
mote from  0,  and  nearer  C  or  D.  Though  the  penumbra  be 
continually  increasing  in  diameter,  according  to  its  length,  or 
the  distance  of  the  moon  from  the  earth,  still,  under  the  most 
favorable  circumstances,  it  falls  on  but  about  half  of  the  illu- 
minated hemisphere  of  the  earth.  Hence,  by  half  the  in- 
habitants on  this  hemisphere,  no  eclipse  will  be  seen, 

5.  Fig.  163  represents  an  ecHpse  of  the  moon.  The  instant 
the  moon  enters  the  earth's  shadow  at  x,  it  is  deprived  of  the 
sun's  light,  and  is  eclipsed  to  all  in  the  unilluminated  hemi- 


Explain  Fig.  162.  When  is  an  eclipse  called  annular  ?  Explain  by  Fig- 
163.    What  is  a  penumbra  ? 


ASTRONOMY. 


291 


s\)\\cvc  o(  ihc  vixvih.  }\cnc{\  cclipsos  of  ihc  Tnoon  arc  visible 
to  at  least,  twice  as  many  inliabilaiits  as  those  of  the  sim  can 


Fig.  1()3. 


B 


be ;  generally  the  proportion  is  much  greater.  Thus,  the  in- 
habitants at  a  particular  place,  as  Boston,  see  more  eclipses  of 
the  moon  than  of  the  sun. 

6.  The  reason  why  a  lunar  eclipse  is  visible  to  all  to  whom 
the  moon  at  the  time  is  visible,  and  a  solar  one  is  not  so  to  all  to 
whom  the  sun  at  the  time  is  visible,  may  be  seen  from  the  na- 
ture of  these  eclipses.  We  speak  of  the  sun's  being  eclipsed  ; 
but,  properly,  it  is  the  earth  which  is  eclipsed.  No  change 
takes  place  in  the  sun ;  if  there  were,  it  would  be  seen  by  all 
to  whom  the  sun  is  visible.  The  sun  continues  to  diffuse  its 
beams  as  freely  and  uniformly  at  such  times  as  at  others.  But 
these  beams  are  intercepted,  and  the  earth  is  eclipsed  only 
where  the  moon's  shadow  falls,  that  is,  on  only  a  part  of  a 
hemisphere.  In  eclipses  of  the  moon,  that  body  ceases  to  re- 
ceive light  from  the  sun,  and,  consequently,  ceases  to  reflect 
it  to  the  earth.  The  moon  undergoes  a  change  in  its  ap- 
pearance ;  and,  consequently,  this  change  is  visible  at  the  same 
time  to  all  to  whom  the  moon  is  visible ;  that  is,  to  a  whole 
hemisphere  of  the  earth. 

7.  The  earth's  shadow  (like  that  of  the  moon)  is  encom- 
passed by  a  penumbra,  C  R  S  D,  which  is  faint  at  the  edges 
towards  R  and  S,  but  becomes  darker  towards  F  and  G.  The 
shadow  of  the  earth  is  but  little  darker  than  the  region  of  the 
penumbra  next  to  it.  Hence  it  is  very  difficult  to  determine 
the  exact  time  when  the  moon  passes  from  the  penumbra  into 


Why  are  eclipses  of  the  moon  visible  to  more  inhabitants  than  those  of 
the  sun  ?  Why  is  a  lunar  eclipse  visible  to  all  to  whom  the  moon  is  visi- 
ble at  the  time  ?  What  is  said  of  the  earth's  shadow  ?  Explain  by  the 
figure. 


298 


NATURAL  PHILOSOPHY. 


the  shadow,  and  from  the  shadow  into  the  penumbra ;  that  is, 
when  the  echpse  begins  and  ends.  But  the  beginnmg  and 
ending  of  a  solar  echpse  may  be  determined  iihnost  mstan- 
taneously.  i  x 

8  The  diameters  of  the  sun  and  moon  are  supposed  to  be 
divided  into  12  equal  parts,  called  digits.  These  bodies  are 
said  to  have  as  many  digits  echpsed  as  there  are  oi  those  parts 
involved  in  darkness. 

9.  There  must  be  an  echpse  of  the  sun  as  often,  at  least,  as 
one  of  the  moon's  nodes  comes  between  the  sun  and  the  earth. 

10.  The  greatest  number  of  both  solar  and  lunar  eclipses 
that  can  take  place  during  a  year  is  seven.  The  usual  number 
is  four— two  solar  and  two  lunar. 

11.  A  total  eclipse  of  the  sun  is  a  very  remarkable  phe- 
nomenon. 

12.  June  16,  1806,  a  very  remarkable  total  eclipse  took 
place  at  Boston.  The  day  was  clear,  and  nothing  occurred  to 
prevent  accurate  observation  of  this  interesting  phenomenon. 
Several  stars  were  visible  ;  the  birds  were  greaily  agitated  ;  a 
gloom  spiead  over  the  landscape,  and  an  indescribable  sensa- 
tion of  fear  or  dread  pervaded  the  breasts  of  those  who  gave 
themselves  up  to  the  simple  effects  of  the  phenomenon,  without 
havino'  their  attention  diverted  by  efforts  of  observation.  The 
first  |leam  of  light,  contrasted  with  the  previous  darkness, 
seemed  like  the  usual  meiidian  day,  and  gave  indescribable 
life  and  joy  to  the  whole  creation.  A  total  echpse  of  the  sun 
can  last  but  little  more  than  three  minutes.  An  annular  echpse 
of  the  sun  is  still  more  rare  than  a  total  one. 


OF  TIME. 

338.  When  time  is  calculated  by  the  sun,  it  is  called 
solar  time,  and  the  year  a  solar  year ;  but  when  it  is 
calculated  by  the  stars,*  it  is  called  sidereal  time,  and 

*  The  solar  year  consists  of  365  days,  5  hours,  48  minutes,  and  48  sec- 

Into  what  are  the  diametei-s  of  the  sun  and  moon  supposed  to  be  divi- 
ded ?  How  many  digits  are  these  bodies  said  to  have  eclipsed  ?  How 
often  must  there  be  an  eclipse  of  the  sun  ?  What  is  the  greatest  number, 
of  both  lunar  and  solar  eclipses,  that  can  take  place  in  a  year?  What  is 
the  usual  number  ?    What  is  said  of  the  eclipse  of  the  sun  in  1806  ? 

338.  What  is  time  called  when  calculated  by  the  sun  ?  What  is  side- 
real  time  ?    How  much  longer  is  the  sidereal  year  than  the  solar? 


ASTRONOMY.  299 

the  year  a  sidereal  year.     The  sidereal  year  is  20 
minutes  and  24  seconds  longer  than  the  solar  year. 
1.  A  soLir  year*  is  measured  from  the  time  the  earth 

onds ;  but  our  common  reckoning  gives  .365  days  only  to  the  year.  As 
the  ditlcrenco  amounts  to  nearly  a  quarter  of  a  day  every  year,  it  is  usual, 
every  fourth  year,  to  add  a  day.  Every  fourth  year,  the  Romans  reck- 
oned the  6th  of  the  calends  of  March,  and  the  following  day  as  one  day  ; 
which,  on  that  account,  they  called  bissextile,  or  twice  the  6th  day  ;  whence 
we  derive  the  name  of  bissextile  for  the  leap  year,  in  which  we  give  to 
February,  for  the  same  reason,  29  days  every  fourth  year. 

*  As  it  may  be  interesting  to  those  who  have  access  to  a  celestial  globe, 
to  know  how  to  find  any  particular  star  or  constellation,  the  following  di- 
rections are  subjoined  : 

There  is  always  to  be  seen,  on  a  clear  night,  a  beautiful  cluster  of  seven 
brijliant  stars,  which  belong  to  the  constellation  "  Ursa  Major,''  or  the 
Great  Bear.  Some  have  supposed  that  they  will  ^tly  represent  a  plough  ; 
others  say  that  they  are  more  like  a  wagon  and  horses,  the  four  stars  rep- 
resenting the  body  of  the  wagon,  and  the  other  three  the  horses.  Hence 
they  are  called  by  some  the  plough,  and  by  others  they  are  called 
Charles's  wain,  or  wagon. 

Fig.  164  represents  these  seven  stars :  ahdg  pig, 
represent  the  four,  and  ezB  the  other  three  p 
stars.    Perhaps  they  may  more  properly  be  .  ^ 

called  a  large  dipper,  of  which  ezB  represent 
the  handle.  If  a  hne  be  drawn  through  the 
stars  b  and  a,  and  carried  upwards,  it  will  pass 
a  Httle  to  the  left,  and  nearly  touch  a  star  rep- 
resented in  the  figure  by  P.  This  is  the  polar 
star,  or  the  north  pole  star ;  and  the  stars  b  i 
and  a,  which  appear  to  point  to  it,  are  called 

the  pointers,  because  they  appear  to  point  to  B  /  ^  G  3»  ^ 
the  polar  star.  /  g 

The  polar  star  shines  with  a  steady  and 
rather  dead  kind  of  light.  It  always  appears 
in  the  same  position ;  and  the  north  pole  of 

the  earth  always  points  to  it  at  all  seasons  of  the  year.  The  other  stars 
seem  to  move  round  it  as  a  centre.  As  this  star  is  always  in  the  north, 
the  cardinal  points  may  at  any  time  be  found  by  starlight. 

By  these  stars  we  can  also  find  any  other  star  or  constellation. 


How  is  a  solar  year  measured?  What  is  the  length  of  a  solar  year? 
Why  is  a  day  added  every  fourth  year,  to  the  year  ?  How  is  a  sidereal 
year  measured  ? 


300 


NATURAL  PHILOSOPHY. 


sets  out  from  a  particular  point  in  the  ecliptic,  as  an  equinox, 
or  solstice,  until  it  returns  to  the  same  point  again.  A  sidereal 
year  is  measuied  by  the  time  that  the  earth  takes  in  making 
an  entire  revolution  in  its  orbit ;  or,  in  other  words,  from  tlie 
time  that  the  sun  takes  to  return  into  conjunction  with  any 
fixed  star. 

2.  Every  equinox  occurs  at  a  point,  50''  of  a  deg.  ot  the 
c^reat  circle,  preceding  the  place  of  the  equinox,  12  months  be- 
fore ;  and  this  is  called  the  precession  of  the  equinoxes.  It  is 
this  circumstance  which  has  caused  the  change  in  the  situation 
of  the  constellations  of  the  zodiac,  of  which  mention  has  al- 
ready been  made.  . 

3.  The  earth's  diurnal  motion  on  an  inclined  axis,  together 
with  its  annual  revolution  in  an  elliptic  orbit,  occasions  so  much 
comphcation  in  its  motion,  as  to  produce  many  irregularities ; 
therefore  true  equal  time  cannot  be  measured  by  the  sun.  A 
clock,  which  is  always  perfectly  correct,  will,  in  some  parts  of 
the  year,  be  before  the  sun,  and,  in  other  parts,  after  it.  There 
are  but  four  periods  in  which  the  sun  and  a  perfect  clock  will 
agree:  these  are  the  loth  of  April,  the  16th  of  June,  the  23d 
of  Auo'ust,  and  the  24th  of  December. 

4.  the  greatest  difference  between  true  and  apparent  time 
amounts  to  between  sixteen  and  seventeen  minutes.  Tables  of 
equation  are  constructed  for  the  purpose  of  pointing  out  and 
correcting  these  differences  between  solar  time  and  equal  or 
mean  time,  the  denomination  given  by  astronomers  to  true 
time. 

Thus,  if  we  conceive  a  line  drawn  from  the  star  z,  leaving  B  a  little  to 
the  left,  it  will  pass  through  the  very  brilliant  star  A.  By  looking  on  a 
celestial  globe  for  the  star  z,  and  supposing  the  line  drawn  on  the  globe, 
as  we  conceive  it  done  on  the  heavens,  we  shall  find  the  star  and  its  name, 
which  is  Arcturus. 

Conceiving  another  line  drawn  through  g  and  and  extended  some 
distance  to  the  right,  it  will  pass  just  above  another  very  brilliant  star.  On 
referring  to  the  globe,  we  find  it  to  be  Capella,  or  the  goat. 

In  this  manner,  the  student  may  become  acquainted  with  tne  appear- 
ance of  the  whole  heavens. 


What  is  the  precession  of  the  equinoxes  ?  What  change  has  this  cir- 
cumstance caused,  with  regard  to  the  situation  of  the  constellations  ?  Can 
true  equal  tune  be  measured  by  the  sun  ?  Why  ?  At  what  periods  of  the 
year  do  the  sun  and  a  perfect  clock  agree  ?  What  is  the  greatest  differ- 
ence between  true  and  apparent  time  ? 


ASTRONOMY, 


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302 


NATURAL  PHILOSOPHY. 


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302 


INDEX. 


(The  simple  figures  refer  to  the  articles  :  those  preceded  by  p,  to  the  pages.] 


A. 

Attii.\ctio\,  11. 

Attraction  of  cohesion,  12  ;  of  gravitation,  12. 
Action,  44. 

Archimedes'  discovery  of  specific  gravity,  117  note  ;  screw  of,  127. 

Air,  how  high  it  extends,  &c.,  137  ;  elasticity  of,  142  ;  pressure  of,  p.  141; 

how  it  becomes  a  mechanical  agent,  143. 
Air-pump,  150  ;  experiments  with,  151,  &c. ;  and  instruments  connected 

with,  151,  ^c. 
Acoustics,  152. 

Angle,  right,  obtuse,  and  acute,  46  note;  of  vision,  213  ;  of  incidence  and 

reflection,  46,  47,  and  48. 
Aqueous  humor,  p.  175. 
Amalgam,  p.  200. 
Aurora  Borealis,  p.  210. 
Armature  of  a  magnet,  p.  239. 

Ampere,  his  apparatus  for  illustrating  the  electro-magnetic  rotation,  p.  246. 
Astronomy,  303. 
Aphelion,  313. 

Apogee,  314.  • 
Axis  of  the  planets,  their  inclination,  &c.,  316. 
Aberration  of  light,  p.  287. 

Asteroids,  supposed  to  be  fragments  of  a  large  planet  burst  asunder,  p.  279 
note. 

Atmosphere,  weight  of,  141  note. 

B. 

Brittleness,  18.  x 

Barometer,  145. 

Battery,  electrical,  259. 

Biela's  comet,  p.  285  note. 

Bissextile,  or  leap-year,  338  note,  p.  298. 

c. 

Compressibility,  13. 
Compound  motion,  49. 


3U4 


INDEX. 


Clock,  how  regulated,  77. 

CyUnder,  88  iiote,  p.  62. 

Complex  wheel-work,  p.  65. 

Capillary  tubes  and  attraction,  p.  74  note. 

Capstan,  89. 

Crepusculum.    See  Twilight. 

Crank,  p.  64. 

Caloric,  note,  p.  134. 

Camera  obscura,  p.  157. 

Catoptrics,  207  ;  fundamental  law  of,  p.  IbJ. 

Cornea,  p.  174. 

Crystalline  lens,  p.  175. 

Choroid,  p.  75. 

Chromatics,  240. 

Color,  cause  of,  241-249. 

Compound  galvanic  battery,  p.  222. 

Couroune  des  tasses,  p.  222. 

Calorimotor,  p.  229  note. 

Constellations,  p.  263  note;  ancient,  p.  287. 

Conjunction,  inferior  and  superior,  p.  26o  note 

Characters  used  in  astronomical  works,  p.  262  note. 

Ses  on  the  earth,  p.  271  and  272  ;  of  perpetual  apparition  and 
tation,  p.  288  ;  galvanic,  270. 

Ceres,  p.  279. 

Comets,  331. 

Clusters  of  stars,  287. 

Celestial  globe,  how  used,  p.  299  note. 

D. 

Divisibility,  8.  ,     u  ion 

Density,  p.  23  ;  how  affected  by  heat  and  cold,  18U. 

Ductility,  19. 

Diving-bell,  p.  105. 

Dioptrics,  221 ;  laws  of,  224. 

Dipping  of  magnetic  needle,  286. 

Distance  of  the  planets  from  the  sun,  p.  260. 

Diameter,  46  note;  of  the  sun,  moon,  and  planets,  3U7. 

Diaoronal,  46  note. 

Days  and  nights,  cause  of  their  different  lengths,  p.  271-^74. 
Digits,  p.  298. 
Discharger,  jointed,  260. 

E. 

Extension,  6. 
Expansibility,  14. 
Elasticity,  16. 


INDEX.  305 

Equilibrium  of  fluids,  101. 
Eclio,  how  produced,  164. 
Eye,  its  parts  aud  description  of,  233,  &,c. 

Electricity,  294 ;  vitreous  and  resinous,  or  positive  and  negative,  256 ; 

conductors  of,  251  ;  by  induction  and  transfer,  p.  199  ;  elicited  from 

a  mag^net,  296. 
Electrics,  251. 

Electrical  machine,  264 ;  battery,  259  ;  experiments  with,  p.  202 ;  elec 
trical  bells,  204 ;  electrical  sportsman,  p.  208 ;  electrical  saw-mill, 
p.  211 ;  electrical  animals,  p.  214  and  215. 

Electrometer,  p.  202. 

^^^''295"'^^''^^'^"''  289 ;  principal  facts  relating  to  the  science,  290  and 

Electric  sparks  taken  from  a  magnet,  p.  240  note. 
Electro-magnetic  telegraph,  300. 
Electro-magnetic  multiplier,  294 ;  rotation,  295. 
Earth,  its  diameter,  &c.,  303. 
Ecliptic,  308. 

Earth,  not  a  perfect  sphere  ;  appears  as  a  moon  to  the  inhabitants  of  the 

moon,  &c.,  324  and  325. 
Encke's  comet,  p.  285. 

Eclipses,  337  ;  total  eclipse  of  the  sun  in  1806,  p.  298. 
Equinoxes,  precession  of,  p.  300. 

F. 

Figure,  7. 

Force,  central,  55  ;  centripetal  and  centrifugal,  55. 
Fulcrum,  78. 
Fly-wheel,  p.  63-64. 

Friction,  p.  69  ;  cause  of,  p.  69  ;  how  lessened,  p.  69. 
Fluids,  99  ;  pressure  of,  106. 
Fountain,  how  formed,  130. 

Farraday's  discoveries,  p.  240  note;  his  apparatus  for  exhibiting  the  elec 
tro-magnetic  rotation,  p.  245. 

G. 

Gravity  or  Gravitation,  21;  force  of,  where  greatest,  23;  effect  of,  on 

fluids,  104 ;  specific  gravity,  27 ;  centre  of,  53. 
Governor,  96. 

Glass  chimneys,  how  protected  from  fracture,  p.  139. 
Gasometer,  or  gas  generator,  p.  207  note. 
Gymnotus  Electricus,  p.  214-215  note. 

Galvanism,  267 ;  difference  between  electricity  and  galvanism,  267 ;  its 

effects,  p.  228  ;  history  of,  p.  216  note. 
Galvanic  conductors,  269. 


306 


INDEX. 


Galvanic  circle,  271 ;  effects  of,  how  increased,  272 
Galvanometer,  294. 
Georgium  Sidus,  p.  282  note. 
Galaxy,  333. 

Gibbous,  when  the  moon  appears,  p.  291. 
Great  Bear,  p.  299  note. 

H. 

Heat,  168;  its  effects,  169;  laws  of,  p.  33-34;  f^cts       ^^^^  whei 
greatest  on  the  earth,  320  ;  how  propagated,  98  ;  and  reflected,  174. 
Hydrostatics,  172. 
Hydrostatic  Bellows,  111. 
Hydrostatic  Press,  (Bramah,)  p.  84. 
Hydrometer,  p.  89. 
Hydraulics,  119. 
Hygrometer,  p.  103. 

Harmony,  science  of,  158.  . 
Heavenly  bodies,  why  not  seen  in  their  real  place,  225 ;  their  situation 

must  be  calculated  from  the  centre  of  the  earth,  p.  288. 
Hydrogen  pistol,  334. 
Bydro-electric  current,  p.  240. 
Helix,  296. 
Hesperus,  323. 
Herschel,  p.  282. 
H alley's  comet,  p.  285. 
Harvest  moon,  p.  292. 
Hunter's  moon,  p.  292. 

L 

Impenetrability,  5. 
IndestructibiUty,  9. 
Inertia,  10. 

Incident  motion,  45 ;  incident  ray,  208 

Incidence,  angle  of,  46. 

Inclined  plane,  90. 

Iris,  233. 

Insulated,  252. 

Induction,  electricity  by,  262 


Jupitl^!  327  ;  his  satellites,  eclipses  of,  327  ;  his  belts,  p.  281 

K. 


Kaleidescope,  248. 

Kepler's  Law,  318  ;  illustration  of,  318. 


INDEX. 


307 


L. 

r.ever,  80. 
Liquids,  100. 

Locomotive  steam-engine,  p.  149. 

Lijrlit,  laws  of,  208  ;  composed  of  different  colors,  243  ;  its  velocity,  how 

ascertained,  p.  280. 
Luminous  bodies,  192. 

Lens,  227;  various  kinds  of,  p.  169;  focal  distance  of,  229;  effects  of, 

p.  171  vote;  why  used  in  spectacles,  232. 
Leyden  jar,  258  ;  how  silently  discharged,  262. 
Lioluning,  p.  210. 

Lightning  rods,  p.  214 ;  square,  better  than  round  ones,  p.  212 ;  must  not 
be  painted,  p.  212  ;  Dr.  King's  and  Mr.  Quimby's,  211 ;  first  proposed 
by  Franklin,  p.  214. 

Line  of  direction,  p.  47. 

Loadstone,  278. 

Lucifer,  323  note. 

Longitude  ascertained  by  eclipses  of  Jupiter's  satellites,  32  7. 

M. 

Matter,  definition  and  properties  of,  3  ;  all  matter  attractive,  21 
Mobility,  15. 
Malleability,  17. 
Mechanics,  28. 

Motion  29;  uniform,  accelerated,  and  retarded,  38;  compound,  49  ;  cir- 
ciijar,  52  ;  centre  of,  54 ;  axis  of,  52 ;  resultant  motion,  71  ;  when 
imperceptible,  214;  cause  of  in  the  heavenly  bodies,  p.  258 ;  their 
motion  not  uniform,  317. 

Momentum,  42.  ~ 

Magnitude,  centre  of,  p.  41. 

Mechanical  powers,  78-97. 

Medium,  93  and  198. 

Main-spring  of  a  watch,  95. 

Magdeburgh  cups,  oi  hemispheres,  p.  112. 

Mirrors,  plain,  concave,  and  convex,  216;  laws  of  reflection  from,  p.  163, 
&c. ;  concave,  why  they  magnify,  p.  160  ;  convex,  why  they  diminish] 

Microscope,  single,  235  ;  and  double,  236  ;  solar,  237. 
Magic  lantern,  238. 
Multiplying-glass,  247. 

Magnetism,  278  ;  how  it  resembles,  and  differs  from  electricity,  284  note; 

communicated  by  electricity,  293  and  298.  * 
Magnet,  properties  of,  279;  polarity  of,  280;  metnods  of  supporting,  p. 

233  ;  iti  powers,  how  increased,  282 ;  horse-shoe  magnet,  p.  235 ; 


308 


INDEX. 


artificial  magnete,  how  made,  p.  239  note;  ma^ets  made  by  elec- 
tricity, 298. 

Mariners  compass,  p.  237  and  238. 

Mars,  326. 

Mercury,  322. 

Meteoric  stones,  p.  279  note. 
Milky  way,  333. 
Moon,  335. 

N. 

Natural  philosophy,  definition  of,  1  ;  principal  branches  of,  9 
Non-electrics,  251. 
JNort  hern  Hghts,  p.  210. 

0. 

Oil,  effects  of,  on  waves,  124  note. 

Optics,  192. 

Optic  nerve,  p.  176. 

(Ersted's  discoveries,  p.  240. 

Orbits  of  the  planets,  306;  their  inclination  to  each  other,  311. 

Opposition,  314  note. 

P. 

Pei-pendicular,  p.  37  note. 
Parallelogram,  p.  37  note. 
Parabola,  64. 

Projectile,  63  ;  random  of,  65. 

Pendulum,  72  ;  vibrations  of,  74,  &c. 

Pulleys,  84 ;  fixed  and  moveable,  84  ;  practical  use  of,  87. 

Pinion,  p.  65. 

Pyronomics,  168. 

Pyrometer,  177. 

Pupil  of  the  eye,  p.  174. 

Prism,  242.  .  ,       r        ^       on  -  . 

Planets  of  the  solar  system,  304 :  how  distinguished  from  stars  30o  m- 
terior  and  exterior,  inferior  and  superior,  p.  260  note;  mhabited,  p 
271  note. 

Phosphor,  323  note. 

Perihelion,  313  ;  of  the  planets,  in  what  sign,  315. 
Perigee,  314. 
Pallas,  279  note. 

Pole  or  Polar  star,  p.  288  ;  how  to  find,  p.  299  note. 
Parallax,  234 
Porosity,  p.  23. 

Q. 

Quadrature,  p.  290 


INDEX. 


309 


R. 

Rarity,  p.  24. 

Reaction,  44. 

Reflected  motion,  45. 

Radii,  p.  37  note.  > 

Radius-vector,  318  note.  \ 

Reflection,  angle  of,  47. 

Reflecting  and  refracting  substances,  p.  151. 
Reflected  ray,  p.  155  note. 

Retraction  of  light,  221 ;  effects  of,  p.  167  and  p.  171 :  laws  of,  231 
Retina,  p.  171. 

Rainbow,  how  produced,  245. 
Resinous  electricity,  255. 

Revolution,  annual,  of  the  planets,  p.  260  ;  around  their  axes,  p.  260 
Rays,  converging,  11)5 ;  diverging,  194 ;  oblique  and  vertical,  effects  of,  320. 
Receiver  of  an  air-pump,  150. 


Square,  p.  38  note. 
Screw,  92. 

Specific  gravity,  27;  of  bodies,  how  ascertained,  116;  standard  of,  115; 

table  of  specific  gravities,  p.  87  note. 
Springs,  how  formed,  128. 
Siphon,  131. 

Sound,  153  ;  produced  by  strings,  157 ;  velocity  of,  163  ;  of  the  human 

voice,  how  produced,  167;  stethescope,  162. 
Sonorous  bodies,  155. 
Steam,  elastic  force  of,  185. 

Steam-engine,  187  ;  moving  part  of,  p.  143  ;  inventors  and  improvers  of, 
p.  145  note;  Watt's  steam-engine,  p.  146;  locomotive  steam-engine 
p.  149.  ^  *  ' 

Shadows,  &c.,  202. 

Sclerotica,  p.  1 76. 

Sky,  cause  of  its  blueness,  p.  186  note. 

Spiral  tube,  p.  206. 

Silurus  Electricus,  p.  214  note. 

Solar  system,  304 ;  relative  size  of  bodies  belonging  to,  307 ;  tables  of  p. 

301  and  p.  302.  ' 
Stars,  p.  258  ;  how  distinguished  from  planets,  305  ;  classed  into  six  mag- 

nitudes,  332  ;  never  seen  in  their  true  situation,  p.  287. 
Size,  relative,  of  heavenly  bodies,  307. 
Seasons,  cause  of  the  variations  of,  p.  271. 
Sun,  its  size,  diameter,  &c.,  321. 
Saturn,  328. 


310 


INDEX. 


T. 

Tables,  p.  301  and  p.  302. 
Tenacity,  20. 
Tackle  and  fall,  p.  61. 
Toggle-joint,  91. 
Tantalus'  cup,  p.  95. 
Thermometer,  p.  103. 

Transparent  and  translucent  substances,  p.  150-151. 

Telescopes,  &c.,  239  ;  refracting  and  reflecting,  p.  180  and  p.  182. 

Transfer,  electricity  by,  263. 

Thunder-house,  p.  209. 

Torpedo,  p.  214  note. 

Thermo-electric  current,  302  note* 

Tropic,  p.  272. 

Tangent,  p.  37  note. 

Transit  of  Mercury  and  Venus,  p.  278. 

Telescopic  stars,  332. 

Tides,  336. 

Time,  solar  and  sidereal,  338 ;  true  and  apparent,  difierence  between, 
p.  300. 

Twilight,  how  caused,  p.  168. 

u. 

Universal  discharger,  265. 

Uranus,  329. 

Ursa  Major,  p.  299  note. 

V. 

Velocity,  32  ;  of  a  current,  how  ascertained,  123. 
Vibrations  of  a  pendulum,  74. 
Vacuum,  p.  110  note. 
Ventriloquism,  p.  131-132. 
Vision,  angle  of,  213. 

Vitreous  humor,  p.  175  ;  vitreous  electricity,  255. 

Voltaic  electricity,  or  galvanism,  267 ;  difference  between  this  and  com- 
mon electricity,  276. 
Voltaic  battery,  274 ;  effects  ot,  &c.,  p.  221-222,  p.  226  note. 
Voltaic  pile,  273. 
Venus,  322. 
Vesper,  p.  277  note. 
Vesta,  p.  279. 


INDKX. 


311 


W. 

Watfli,  how  it  (lilltTs  from  a  clock,  p.  53  notr. 
NN'liool  and  axle,  88. 
VVod^ro,  91. 

Water,  compressible,  p.  74  note;  instruments  for  raising,  125;  how  it  bo- 
comes  a  meclianical  agent,  13)2. 
AVater-level,  103. 
Waves,  how  formed,  124. 

^^  at er- wheels,  oversliot,  undershot,  and  breast,  125. 
Wind,  149. 

^^'  li  ispering-sralleries,  1 65. 

Y. 

Year,  solar  and  sidereal,  336. 

z. 

Zodiac,  309  ;  signs  of,  310 
Zodiacal  light,  p.  276. 


QUESTIONS  FOR  REVIEW  AND  EXAMESfATION. 


DIVISIONS  OF  THE  SUBJECT. 
Art.  1.— What  is  Natural  Philosophy  ?  ' 
Art.  2— What  are  the  principal  branches  of  Natural  Philosophy  ?  What 
is  Mechanics?  Of  what  does  Pneumatics  treat?  Of  what  does 
Hydrostatics  treat?  Hydraulics?  Acoustics?  Pyronomics?  Op- 
tics? Astronomy  ?  Electricity  ?  Of  what  is  Galvanism  a  branch? 
Of  what  does  Magnetism  treat?  Electro-Magnetism?  Magneto- 
Electricity  ? 

OF  MATTER  A>T)  ITS  PROPERTIES. 

3.— \Miat  is  Matter? 
Art.  4.— How  many  essential  properties  of  matter  are  there  ?  ^^^lat  are 
they'?    Whyarethev  called  essential  properties?       hat  other 
properties  exist  in  different  bodies?    Why  are  they  called  acci- 
dental properties?    Are  color  and  weight  essential  or  accidental 
properties  ?  Whv  ?   What  terms  are  used  in  PhOosophy  to  express 
the  state  in  which  matter  exists  ?    What  foUows  from  this  ? 
Ficr  1.  Explain  the  principles  which  Fig  1  illustrates.       hat  example 
can  you  give  to  prove  the  impenetrability  of  water  ?    ^^  hat  of  the 
air  1    WTiat  of  solids  ? 
j  Art.  5.— What  is  meant  by  Impenetrability  ?    Does  impenetrability  be- 
!  loner  to  fluids  ?    Why  do  fluids  appear  less  impenetrable  than  sohd 

bodies  ?  What  is  supposed  to  be  the  form  of  the  particles  of  fluids  ? 
Art  6.— What  is  meant  bv  Extension  ?  What  terms  are  used  to  ex- 
press the  size  of  a  b^>dy  ?  What  is  length  ?  Breadth  ?  Height, 
depth,  or  thickness?  What  is  the  difference  between  height  and 
depth  ? 

Art  7. -What  is  meant  bv  Figure  ?    May  bodies  be  of  the  same  shape 

or  figure  and  of  different  dimensions  ?    Give  an  example.  hat 

constitutes  figure? 
8.— What  is  meant  bv  Divisibility?    Is  there  any  known  limit  to 

the  divisibility  of  matter?  Mention  some  examples  of  the  extreme 

divisibilitv  of  matter. 
Art  9.— What  'is  meant  bv  the  Indestructibility  of  matter  ?    :\Iay  it  be 

changed  in  form  and  in  external  appearance  ?    Give  examples  oi 

such  changes. 


aUKSTIONS   FOR    EXAMINATION.  313 


I  ()._^^  h,,t  ,8  meant  by  Inertia  ?  Can  a  body  at  rest  put  itself  in 
motion  ?  Can  a  body  in  motion  stop  itself?  When  a  stone  or  ball 
18  thrown  from  the  hand,  how  many  forces  continually  operate 
to  stop  it?  What  are  they?  How  could  a  body  in  motion  be 
made  to  move  forever? 

Fig.  2. 


Fig.  2.     Explain  the  apparatus  for  illustrating  inertia,  as  shown  in 
Fig.  2. 


4.RT.  H.-What  is  attraction ?  Is  every  portion  of  matter  attracted  by 
every  other  portion  of  matter?  How  does  this  attraction  in- 
crease and  diminish? 

^KT.  12._How  many  kinds  of  attraction  are  there  belonging  to  all 
matter?  What  is  the  attraction  of  gravitation,  or  gravity? 
Wha  ,s  the  attraction  of  cohesion,  or  cohesive  attraction  ? 
What  causes  a  stone  to  fall  to  the  ground?  By  what  are  the 
particles  which  compose  the  stone  held  together?  Of  what  is 
matter  composed?  Is  the  cohesive  power  which  unites  them 
.he  same  m  all  bodies  ?  How  may  cohesive  attraction  be  illus- 
trated? Do  the  particles  of  matter  in  bodies  absolutely  touch 
each  other?  What  are  the  spaces  between  them  called?  What 
do  you  understand  by  density  ?  What  by  rarity  ? 
Art.  18--What  is  compressibility?    Are  all  substances  susceptible 

Art.  14-.— What  is  expansibiUty  ? 
Art.  15.— What  is  mobility? 

Art.  16.-What  is  elasticity?    What  substance  possesses  this  property 

m  a  remarkable  degree  ?  ^  r  j 

Art.  17.-What  is  malleability?    Does  this  property  belong  to  all  the 

metals?    What  metal  possesses  it  in  the  highest  degree? 
Art.  1 8.— What  is  brittleness  ?    What  bodies  are  most  brittle  ? 
Art.  1 9— What  is  ductility  ?    Which  is  the  most  ductile  of  the  metals  ? 
Art.  20.-What  is  tenacity  ?  Which  is  the  most  tenacious  of  the  metals  ? 

14~~ 


314 


aUESTIONS    FOR  EXAMINATION. 


OF   GRAVITY  AND  WEIGHT. 

Art  21  What  do  you  understand  by  the  term  gravity?    Do  all  bodies 

possess  this  attraction  ?  To  what  is  its  force  proportional  ?  If  a 
body  be  unsupported,  will  it  remain  stationary?  Why  will  it 
fall? 

Art.  22.— What  causes  weight?    When  you  say  that  a  body  weighs  an 

ounce,  what  do  you  mean  by  it  ?    What,  therefore,  is  weight  ? 
Art.  23.— Where  is  the  force  of  gravity  greatest?    How  does  it  change  ? 

How  does  it  decrease  above  the  surface  of  the  earth?  How 
below  ? 

Art.  24:.— In  what  direction  will  a  falling  body  approach  the  surface  of 
the  earth  ?    Will  the  lines  of  suspension  of  different  bodies  ever 
be  parallel  ?    Where  will  they  meet,  if  sufficiently  produced  ? 
Fig.  3.     Explain  Fig.  3. 

Art.  25.— Will  all  bodies  at  equal  distances  from  the  earth,  fall  to  it  in 
the  same  time?    Why  not?    What  bodies  fall  fastest?  To 
what  is  the  resistance  of  the  air  proportional  ? 
Art.  26.— In  what  proportion  are  all  substances  influenced  by  gravity? 

Is  air  affected  by  it?  How  far  does  the  air  extend  above  the 
surface  of  the  earth?  What  causes  the  air  to  be  more  dense  at 
the  surface  of  the  earth?  What  causes  this  pressure?  Why 
does  not  the  air  fall  to  the  earth  like  other  bodies?  Where  is 
the  air  heaviest  ?  What  effect  have  gravity  and  elasticity  upon 
the  air  ? 

Art.  2T.— What  is  specific  gravity?  Illustrate  this.  Does  the  attrac- 
tion of  the  earth  cause  all  bodies  to  fall?  What  bodies  will  fall  ? 
What  rise  ?  How  does  the  air  cause  them  to  rise  ?  Why  do 
not  cork  and  other  light  bodies  sink  in  water?  Explain  the 
principle  upon  which  balloons  rise.  What  effect  has  gravity  on 
bodies  lighter  than  the  air?  What  effect  on  bodies  of  equal 
weight?  What  effect  on  those  that  are  heavier?  What  affects 
the  rapidity  of  their  descent?  To  what  is  the  resistance  of  the 
air  proportioned  ? 

MECHANICS  LAWS  OF  MOTION. 

Art.  28, — What  is  Mechanics? 

Art.  29.— What  is  motion?    Why  cannot  a  body  put  itself  in  motion? 

Why  cannot  a  body  stop  itself  when  in  motion  ? 
Art.  30. — What  is  force?    What  is  resistance? 

Art.  31- — When  is  the  motion  of  a  body  in  a  straight  line?    In  what 

direction  will  it  move  ? 
Art  3  2. — What  is  meant  by  velocity  ? 


aUESTIONS   FOR  EXAMINATION. 


315 


Art.  3  3* — To  what  is  the  velocity  of  a  moving  body  proportional? 

Art.  3  I:. — How  is  the  velocity  of  a  rnovin^r  body  determined?  If  one 
body  go  throii<^li  six  miles  in  an  hour,  and  another  twelve,  how 
does  tiie  velocity  of  the  latter  compare  with  that  of  the  former? 
What  is  meant  by  absolute  velocity  ?  Give  an  example.  When 
is  the  velocity  of  a  body  termed  relative  ?    Give  an  example. 

Art.  35. — How  is  the  velocity  of  a  body  measured?    Illustrate  this. 

Art.  36. — How  do  you  ascertain  the  time  employed  by  a  body  in  mo- 
tion ?    Illustrate  this. 

Art.  3T. — How  can  you  ascertain  the  space?    Illustrate  this. 

Art.  38. — How  many  terms  are  applied  to  motion  to  express  its  kind? 

What  are  they  ?  What  is  uniform  motion  ?  Accelerated  ? 
Retarded  ? 

Art.  39. — How  is  uniform  motion  produced  ?  Why  is  not  a  ball  struck 
by  a  bat,  or  a  stone  thrown  from  the  hand,  an  instance  of  uni- 
form motion  ?    How  can  it  be  made  an  instance  ? 

Art.  40. — How  is  accelerated  motion  produced  ?  Give  an  instance  of 
accelerated  motion.  How  far  does  a  stone  fall  the  first  second 
of  time  ?  The  second  ?  Third  ?  Fourth  ?  How  can  you  mea- 
sure the  height  of  a  building,  or  the  depth  of  a  well  ? 

Art.  4:1. — How  is  retarded  motion  produced?  Give  an  example.  How 
does  the  time  of  the  ascent  of  a  body  thrown  perpendicularly 
upwards,  compare  with  that  of  its  descent Why  cannot  per- 
petual motion  be  produced  ? 

Art.  43. — What  is  the  momentum  of  a  body?  How  can  the  momen- 
tum of  a  body  be  ascertained  ?  Does  the  quantity  of  motion 
communicated  to  a  body  affect  the  duration  of  the  motion  ?  If 
but  little  motion  is  communicated,  how  will  the  body  move?  If 
a  great  degree  ?  How  long  will  the  motion  continue  ?  How 
can  a  light  body  be  made  to  have  a  greater  momentum  than  a 
heavy  one  ?    Give  an  instance  of  this. 

Art.  43.— What  is  meant  by  action?    Reaction?    Illustrate  this. 

Art.  44. — How  do  action  and  reaction  compare  ? 

Fig.  4.     Explain  Fig.  4. 


Fig.  3.  Fig.  4. 


316  auESTioxs  for  examination. 


Fig.  5.     Explain  the  experiment  of  Fig.  5. 

Fig.  6.  Explain  the  experiment  of  Fig.  6.  Upon  what  principle  do  birds 
fly?  Explain  how.  Upon  what  principle  do  fishes  swim? 
Upon  what  principle  do  boats  move  upon  the  water?  Explain 
how. 

Art.  45. — How  may  motion  be  caused?  When  caused  by  action,  what 
is  it  called  ?    When  caused  by  reaction,  w^hat  is  it  termed  ? 

Art.  46. — What  is  the  angle  of  incidence?  What  is  an  angle?  Upon 
what  does  the  size  of  an  angle  depend?  What  is  a  circle? 
What  are  radii?  ^\^lat  lines  in  Fig.  8  are  radii?  What  are 
diameters?  In  Fig.  8,  what  line  is  the  diameter?  How  is  the 
circumference  of  all  circles  divided  ?  Into  how  many  parts  does 
the  diameter  of  a  circle  divide  it  ?  How  are  all  angles  mea- 
sured ? 

Fig.  8.  Illustrate  this  by  Fig.  8.  How  many  degrees  do  right  angles 
contain  ?  Acute  ?  Obtuse  ?  Illustrate  these  angles  by  Fig.  8. 
What  is  a  perpendicular  line  ?  What  line  is  perpendicular  in 
Fig.  8  ?  W^hat  is  a  tangent  ?  What  line  is  a  tangent  in  Fig.  8  ? 
What  is  a  square  ?  What  is  a  parallelogram  ?  A  rectangle  ? 
What  is  a  diagonal  ?  What  lines  are  diagonals  in  Figs.  9,  10, 
and  II? 

Fig.  7.     Explain  the  angle  of  incidence  by  Fig.  7. 

Art.  4T .—What  is  the  angle  of  reflection  ?    Illustrate  this  by  Fig.  7. 

Art.  48. — How  do  the  angles  of  incidence  and  reflection  compare  with 
each  other  ?  Illustrate  this  by  Fig.  1,  What  follows  from  w^hat 
has  been  stated  with  regard  to  the  angles  of  incidence  and 
reflection  ? 

Art.  49. — What  is  compound  motion  ? 

Art.  50. — In  what  direction  will  a  body,  struck  by  two  equal  forces  m 
opposite  directions,  move  ? 

Art.  51. — When  struck  by  two  forces  inclined  to  each  other,  how  will 
it  move  ?    What  is  this  line  called  ? 

Fig.  9.  Illustrate  these,  first,  by  Fig.  9,  w^hich  represents  a  ball  struck 
by  two  equal  forces  in  different  directions. 

Fig.  10.  Second,  by  Fig.  10,  which  represents  a  ball  struck  by  two  un- 
equal forces,  acting  at  right  angles. 

Fig.  11.  Third,  by  Fig.  11,  where  the  forces  operate  in  the  direction  of 
an  acute  angle.  Fourth,  by  Fig.  11,  where  the  forces  operate 
in  the  direction  of  an  obtuse  angle. 

Art.  52. — What  is  circular  motion?   How  is  it  caused?    Illustrate  this. 

Art.  53. — How  many  different  centres  are  there  which  require  to  be 
noticed  ?    Define  each  of  them. 


aUESTIONfS   FOR  EXAMINATION.  317 


Art.  s  i* — Is  the  centre  or  axis  of  motion  supposed  to  be  at  rest,  or  does 
it  move  ?  To  what  do  the  terms  centre  of  motion  and  axis  of 
motion  relate  ? 

Art.  55. — What  are  the  two  forces  called  which  produce  circular  mo- 
tion ?  What  is  the  name  of  each  ?  What  do  the  words  centrip- 
etal and  centrifugal  mean  ? 

Art.  56. — Define  a  centripetal  force.    Also  a  centrifugal  force. 

Art.  5T, — If  the  centrifugal  force  be  destroyed,  to  what  point  will  the 
body  tend  ?  What  would  be  its  direction  if  the  centripetal  force 
were  destroyed  ?    Give  an  example. 

Art.  58. — What  parts  of  a  body  move  with  the  greatest  velocity?  In 
what  proportion  does  the  velocity  of  all  the  parts  diminish  ? 

Fig.  12.  What  does  Fig.  12  represent  ? 

Art.  59. — What  follows,  with  regard  to  the  motion  of  the  earth,  from 

the  illustration  of  Fig.  12  ? 
Art.  60. — Of  what  is  curvilinear  motion  always  the  result  ?    Why  ? 


Fig.  5.  Fig.  6.  Fig.  7. 


Fig.  11.  Fig.  12. 


31S 


Art.  61. — How  many  forces  act  upon  a  ball  thrown  in  a  horizontal  di- 
rection ?    What  are  they  ?    Why  do  bodies  fall  to  the  ground  ? 

Art.  62. — Does  the  force  of  gravity  either  increase  or  decrease  the  force 
of  projection  ?    Give  an  illustration. 

Fig.  13.   Explain  Fig.  13. 

Art.  63. — What  is  a  projectile?  What  lines  do  projectiles  describe? 
From  what  cause  ?    Give  the  illustration. 

Fi^.  14.  Explain  Fig.  14.  How  great  is  the  resistance  of  the  air  calcu- 
lated to  be  to  a  cannon-ball  of  two  pounds  weight,  with  the 
velocity  of  2000  feet  in  a  second  ? 

Art.  64:. — When  a  body  is  thrown  horizontally,  or  upwards  or  down- 
wards obliquely,  in  what  curve  will  it  move  ?  In  what  line  will 
it  move  when  thrown  upwards  or  downwards  perpendicularly  ? 

Fig.  15.   Explain  Fig.  15. 

Art.  6o.— What  is  the  random  of  a  projectile?    At  what  angle  does 

the  greatest  random  take  place  ? 
Fig.  16.   Explain  Fig.  16. 

Art.  66.— When  the  centre  of  gravity  of  a  body  is  supported,  will  the 
body  stand  or  fall  ?  What  if  the  centre  be  unsupported  ?  What 
is  a  line  of  direction  ? 

Art.  67.— What  is  the  base  of  a  body? 

Fig.  17.   Explain  the  base  in  Fig.  17. 

Fig.  18.   Explain  what  is  meant  by  the  base  in  Fig.  18. 

Art.  68. — If  the  line  of  direction  falls  within  the  base,  will  the  body 
stand  or  fall  ?    Give  an  illustration. 

Art.  69. — What  shaped  bodies  are  easily  overturned?  What  bodies 
must  stand  more  firmly  than  others  ?  Why  ?  Why  do  bodies 
which  have  a  narrow  base  overturn  more  easily  than  those  which 
have  broad  bases?  Why  can  a  person  earn,'  two  pails  of  water 
more  easily  than  one?  W^hy  is  a  pyramid  the  firmest  of  all 
structures  ? 

Art.  to. — If  two  bodies  of  equal  weight  are  fastened  together,  where  is 

the  centre  of  gravity?    If  one  be  heavier  than  the  other? 
Fig.  19.  What  does  Fig.  19  represent? 
Fig.  20.  What  does  Fig.  20  represent  ? 

Fig.  21.   What  does  Fig.  21  represent  ?    Explain  each  one  separately. 

RESULTAXT  MOTION   AXD  THE  PENDULUM. 

Art.  T1. — What  is  resultant  motion?    Give  the  examples  of  this  kind 

of  motion. 
Fig.  22.  Explain  Fig.  22. 


QUESTIONS   FOR   KX  AMIN  ATION. 


Art.  '7  2. — Of  what  does  a  pendulum  consist?  By  whom  was  the  pen- 
dulum invented?  What  led  him  to  the  discovery?  By  whom 
was  the  principle  of  gravitation  discovered?  What  led  him  to 
the  discovery? 

Art.  73. — What  are  the  movements  of  the  pendulum  called?  What  is 
meant  by  its  arc  ?    What  causes  its  vibrations  ? 

Art.  Ti:. — How  do  the  vibrations  of  pendulums  of  equal  length  com- 
pare? 

Fig.  23.   Explain  this  by  Fig.  23. 

Art.  T 5. — Upon  what  does  the  time  of  the  vibrations  of  a  pendulum 
depend  ? 

Art.  T6. — What  is  the  length  of  a  pendulum  which  vibrates  sixty  times 
in  a  minute  ?  Do  different  situations  affect  the  vibrations  ?  How 
can  a  pendulum  which  vibrates  seconds  at  the  equator  be  made 
to  vibrate  seconds  at  the  poles  ? 


Fig.  13.  Fig  14.  Fig.  15. 


a 


Fig.  16.  Fig.  17  Fig.  18. 


320  auESTioxs  for  examination. 


Art.  it. — How  is  a  clock  regulated?  What  effect  has  the  leugtheniiig 
of  the  pendulum?  The  shortenmg?  What  is  a  clock?  Of 
what  use  is  the  weight?  What  do  the  wheels  show?  Why  do 
clocks  go  slower  in  smnmer  than  in  wrinter  ?  How  does  a  watch 
differ  from  a  clock  ? 

OF  THE   MECHANICAL  POWERS. 

Art.  TS, — What  are  the  mechanical  powers?  How  many  things  are 
to  be  considered  in  order  to  understand  the  power  of  a  machine? 
What  is  the  first  ?  Second  ?  Third  ?  Fourth  ?  Fifth  ?  What 
is  the  power  that  acts?  What  is  the  resistance  to  be  overcome? 
What  is  the  fulcrum  ?    What  is  the  velocity  ? 

Art.  T9, — How  many  mechanical  powers  are  there?   What  are  they? 

Art.  80. — What  is  a  lever  ?  How^  many  kinds  of  levers  are  there  ? 
How^  do  they  differ  ? 

Art.  81. — What  is  a  lever  of  the  first  kind?  What  figure  illustrates 
this? 

Fig.  24.  Explain  it  by  Fig.  24.  To  what  is  the  advantage  gained  by 
this  lever  proportional?  What  follows  from  this?  What  is 
meant  by  an  inflexible  bar?  What  is  a  fundamental  principle 
in  Mechanics  ? 

Fig  25.   Illustrate  this  by  Fig.  25.    Does  this  principle  apply  to  all  the 

mechanical  powers  ? 
Fig.  26.   Explain  the  common  steelyard  in  Fig.  26. 

Fig.  27.  Also  in  Fig.  27.    What  is  a  balance,  or  pair  of  scales?  Give 

some  examples  of  levers  of  the  first  kind. 
Fig.  28.   Explain  Fig.  28. 

Art.  82. — What  is  a  lever  of  the  second  kind?  What  fio;m'e  illustrates 
this  ? 

Fig.  29.  Explain  Fig.  29.  To  what  is  the  advantage  gained  by  this  lever 
proportional?    Give  some  examples  of  level's  of  the  second  kind. 

Art.  83. — What  is  a  lever  of  the  third  kind?  In  what  proportion  must 
the  power  exceed  the  w^eight  in  this  lever  I 

Fig.  30.  Explain  Fig.  30.  Give  some  examples  of  levers  of  the  third 
kind. 

Art.  84. — What  is  a  pulley?    How  many  kiuds  of  pulleys  are  there? 

What  are  they?    What  is  a  fixed  pulley? 
Fig.  31.   Explain  Fig.  31.    What  advantage  is  gained  by  this  pulley? 

What  is  the  use  of  this  pulley?    Upon  what  principle  does  the 

fixed  pulley  operate  ? 
Art.  85. — How  does  the  moveable  pulley  differ  from  the  fixed  pulley? 
Fig.  32.   Explain  Fig.  32. 


Fig.  29.  Fig.  30.  Fig.  31.  Fig.  32. 


14* 


322 


QUESTIONS   FOR  EXAMINATION. 


Art.  86.— How  can  the  power  gained  by  the  use  of  the  moveable  pulley 

be  ascertained?    What  illustration  of  this  is  given? 
Fig.  33.  What  does  Fig.  33  represent?    Explain  it. 

Art.  8T. — Upon  what  principle  do  pulleys  act?  What  advantage  is 
gained  by  the  use  of  pulleys  and  other  mechanical  powers? 
What  are  some  of  the  practical  uses  of  the  pulley?  What  is  a 
tackle  and  fall  ?  Is  there  any  time  or  velocity  gained  by  the 
power  in  the  mechanical  powers?  To  what  is  the  product  of 
the  weight,  multiplied  by  its  velocity,  always  equal  ?  What  rule 
is  given '? 

Art.  88.— Of  what  does  the  wheel  and  axle  consist?  What  is  a  cylin- 
der? What  figures  illustrate  the  wheel  and  axle?  Explain. 
To  what  is  the  advantage  gained  in  proportion  ? 

Fig.  34.   What  does  Fig.  34  represent?    Explain  it. 

Fig.  35.  Wliat  does  Fig.  35  represent  ?    Explain  it. 

Art.  89. — Upon  what  principle  are  the  wheel  and  axle  constructed  ? 

Explain  how.  Upon  what  principle  is  the  capstan  on  board  of 
vessels  constructed  ?  Of  what  does  it  consist  ?  What  other 
things  are  mentioned  as  constructed  upon  this  principle  ?  Are 
wheels  an  essential  part  to  most  machines  ?  Are  they  applied 
in  more  than  oneway?  Wlien  they  are  affixed  to  the  axle, 
in  what  proportion  is  the  power  increased?  What  are  fly- 
wheels, and  for  what  are  they  used  ?  How  are  they  made  to 
revolve?  When  once  set  in  motion,  what  causes  them  to  move 
on  for  some  time  ?  Of  what  service  are  they  in  a  machine  ? 
For  what  are  cranks  sometimes  connected  with  the  axle  of  a 
wheel  ?    How  are  they  made  ? 

Fio-.  36.  What  does  Fig.  36  represent?  Explain  it.  For  what  are  cranks 
often  used  ?  How  does  the  velocity  of  the  wheel  compare  with 
that  of  the  axle  ?  To  what  is  this  velocity  in  proportion  ?  Is 
any  advantage  taken  of  this  in  driving  machinery  where  the 
speed  is  to  be  increased  or  diminished  ?  How  would  rapid  mo- 
tion be  produced  ?    Slow  motion  ? 

Fig.  37.  Explain  Fig.  37.  WHiat  is  the  usual  way  of  transmitting  the 
action  of  the  axles  to  the  adjoining  wheels?  What  are  the  cogs 
on  the  surface  of  the  wheel  called  ?  Those  on  the  axle  ?  What 
is  a  pinion  ? 

Fig.  38.   Explain  Fig.  38.    By  what  are  wheels  sometimes  turned  ? 

What  figure  represents  one  ?    In  what  way  can  the  motion  be 

made  direct  or  reversed  ? 
Fig.  39.   What  does  Fig.  39  represent  ?    Explain  it. 

Fig.  40.  What  does  Fig.  40  represent  ?  Explain  it.  In  what  way  can 
diftbrent  directions  be  given  to  the  motion  produced  by  wheels  ? 


324 


aUESTIONS   FOR  EXAMINATION. 


Fig.  41.  What  does  Fig.  41  represent?  Explain  it. 
Fig.  42.  What  does  Fig.  42  represent  ?    Explain  it. 

Art.  90. — What  is  an  inclined  plane  ?  What  figure  represents  an  in- 
clined plane  ? 

Fig.  43.  Explain  Fig.  43.  To  what  is  the  advantage  gained  by  the  use 
of  the  inclined  plane  in  proportion?  What  follows  from  the 
greater  or  less  inclination  of  the  plane  ?  Give  some  instances 
of  ' the  application  of  the  inclined  plane.  Is  any  time  gained  by 
the  use  of  the  inclined  plane  ?  Upon  what  principle  are  chisels 
and  other  cutting  instruments,  which  are  sloped  tDnly  on  one 
side,  constructed  ? 

Art.  91. — Of  what  does  the  wedge  consist? 

Fig.  44.  What  does  Fig.  44  represent?  Explain  it.  To  what  is  the  ad- 
vantage gained  by  the  wedge  in  proportion  ?  Of  what  use  is 
the  wedge?    Give  some  examples  of  the  wedge. 

Art.  92. — Of  what  does  the  screw  consist?    Of  how  many  parts  is  it 

generally  composed  ?    What  are  they  ? 
Fig.  45.  What  figure  represents  the  screw  and  the  nut  ?    Explain  the 

figure. 

Fig.  46.  How  does  Fig.  45  differ  from  Fig.  46?  Is  the  screw  a  simple  or 
compound  power  ?  How  is  the  power  of  the  screw  estimated  ? 
How  does  the  closeness  of  the  thread  aiiect  the  power?  What 
is  the  use  of  the  screw?  How  can  its  power  be  increased? 
To  what  is  the  screw  applied  ?  What  is  meant  by  friction  in 
machinery  ?  How  many  kinds  of  friction  are  there  ?  What 
are  they?  How  is  the  rolling  friction  produced?  The  sliding? 
Which  is  overcome  with  the  less  difficulty,  the  rolling  or  sliding? 
What  allowance  must  always  be  made,  in  calculating  the  power 
of  a  machine  ?  What  proportion  of  the  power  is  usually  com- 
puted to  be  destroyed  by  friction  ?  Between  which  is  friction 
the  less,  roUing  bodies  or  those  that  slide  ?  What  causes  fric- 
tion ?  In  what  proportion  is  it  diminished  ?  In  what  manner 
can  it  be  lessened  ?  What  is  the  use  of  wheels  ?  In  what  pro- 
portion do  they  overcome  the  obstacles,  such  as  stones,  &c.,  in 
the  road  ?  Why,  in  descending  a  steep  hill,  are  the  wheels  of  a 
carriage  often  locked  ?  How  do  castors,  which  are  put  upon 
furniture,  facilitate  the  moving  of  it  ?  How  is  the  motion  of  all 
bodies  influenced  ? 

Art.  93, — What  is  meant  by  a  medium?    Give  examples. 

Art.  94:. — To  what  is  the  resistance  of  a  medium  in  proportion  ?  What 

illustration  is  given  ?    When  would  a  machine  be  perfect  ? 
Art.  95. — Of  what  does  the  main-spring  of  a  watch  consist?    What  is 

its  use  ?    Does  the  spring  exert  a  stronger  force  when  closely 


325 


coiled,  or  wlien  partly  loosened  ?    What  is  done  in  order  to  cor- 
rect this  inequality  ? 
Fijr.  47.  What  does  Fig.  47  represent  ?    Explain  it. 

Art.  96. — What  is  a  governor  ? 

Fig.  48.  Explain  Fig.  48.    What  is  said  of  the  use  of  the  governor  ? 

Art.  9T. — Of  v^^hat  does  the  knee-joint,  or  toggle-joint,  consist?  In 
Vi^hat  proportion  does  it  increase  in  power? 


Fig.  47. 

A 


Fig.  48 
A  A 


326 


aUESTIONS  FOR  EXAMINATION. 


Fig  49  What  does  Fig.  49  represent  ?  Explain  the  figure.  Give  an 
instance  of  the  operation  of  the  toggle-joint.  What  is  its  use 
in  printing-presses  ? 


HYDROSTATICS. 
Art.  98.— Of  what  does  Hydrostatics  treat? 

Art.  99.— What  is  a  fluid  ?  Does  the  attraction  of  cohesion  have  much 
influence  on  the  particles  of  fluids ?    What  follows  from  this? 

Art.  lOO. — How  do  fluids  differ  from  liquids?  Can  water  be  com- 
pressed ?  What  is  supposed  to  be  the  primary  cause  of  the  fluid 
form  of  bodies  ?  What  effect  has  heat  upon  bodies  ?  What 
illustration  is  given  ?  Why  do  fluids  gravitate  in  a  more  per- 
fect manner  than  solids?  What  is  inferred  from  the  slight 
degree  of  cohesion  in  the  particles  of  fluids?  Why  smooth  ? 
Why  globular  ?  Why  cannot  fluids  be  formed  into  figures,  or 
presei-ved  in  heaps?  What  do  you  understand  by  capillary 
attraction?  Explain  the  reason  of  it.  Explain  why  the  same 
takes  place  in  all  porous  substances.  Explain  all  the  circum- 
stances attending  the  burning  of  a  lamp.  Explain  the  experi- 
ment with  the  glass  plates. 

Art.  lOl.— What  is  meant  by  the  level  or  equilibrium  of  fluids  ?  Have 
all  fluids  a  tendency  to  preserve  this  equihbrium?  What  fol- 
lows from  this  ?  Of  what  is  this  level  or  equilibrium  of  fluids* 
the  natural  result  ?  How  does  the  gravitation  of  solid  bodies 
differ  from  that  of  fluids  ? 

Art.  102. — Do  fluids  of  different  densities  all  preserve  their  own  equi- 
librium ?    What  illustration  is  given  to  prove  this  ? 

Art.  103. — Upon  what  principle  is  a  water-level  constructed?  Of  what 
does  it  consist  ?    For  what  is  it  used  ? 

Fig.  50.     What  figure  represents  a  water-level?    Explain  the  figure. 

Art.  104:. — In  what  manner  do  sohd  bodies  gravitate?  What  is  the 
centre  of  gravity?  What  effect  has  gravity  on  the  particles  of 
fluids? 

Art.  105. — How  is  the  pressure  of  fluids  exerted?    How  long  will  the 

particles  of  fluids  remain  at  rest? 
Fig.  51.     Explain  Fig.  51. 

Fig.  52.  What  does  Fig.  52  represent?  Explain  it.  If  the  equality  of 
the  pressure  be  undisturbed,  what  will  follow?  If  the  fluid  be 
agitated,  when  will  it  again  come  to  a  state  of  rest  ?  How  is 
the  downward  pressure  of  fluids  shown  ?  The  lateral  pressure  ? 
The  upward  pressure  ? 

Art.  1  06.— To  what  is  the  pressure  of  a  fluid  in  proportion  ?  In  what 
direction  is  this  pressure  exerted  ?    What  illustrations  are  given 


aUESTIONS  FOR  EXAMINATION. 

to  prove  lliis  ?  Why  can  a  bottle,  fillod  with  water  or  any  other 
hquid,  be  let  down  to  any  depth  witlioiit  injury  ?  What  ex- 
periment is  mentioned  in  tlie  note  ?  What  opinion  have  some 
philosophers  expressed? 

Art.  lOT.— What  canses  the  lateral  pressure?   What  follows  from  this? 

Fig.  53.     Explain  Fig.  53. 

Art.  108.— Does  the  length  or  the  width  of  the  vessel  in  which  a  fluid 
is  contained  have  any  effect  upon  the  lateral  pressure?  By 
what  is  it  affected? 

Art.  109. — How  does  the  lateral  pressure  on  one  side  of  a  cubical  ves- 
sel compare  with  the  pressure  downwards  ?  How  would  you 
explain  this? 

Art.  1  lO. — What  causes  the  upward  and  downward  pressure  ? 
Fig.  54.     Illustrate  this  by  Fig.  54. 

Art.  111.— Upon  what  does  the  force  of  pressure  depend?    What  is 

meant  by  the  hydrostatic  paradox  ?    What  is  the  use  of  the 

hydrostatic  bellows? 
Fig.  55.     What  figure  represents  the  hydrostatic  bellows?    Explain  the 

figure.    What  is  the  fundamental  principle  of  Mechanics?  Is 

this  the  principle  of  the  hydrostatic  bellows? 

Fig.  49.  Fig.  50.  Fig.  51. 


328 


aUESTIONS  FOR  EXAMIPJATION. 


Fig.  56.  What  does  Fig.  56  represent?  Explain  why  fluids  press  ac- 
cording to  the  heights,  and  not  according  to  the  quantity. 

Art.  1 1 2, — What  fact  is  mentioned  in  this  number  in  regard  to  the  pres- 
sure of  water?  Upon  what  principle  is  Bramah's  hydrostatic 
press  constructed? 

Fig.  57.  What  figure  represents  this?  Explain  the  figure.  To  v;hat 
uses  is  this  press  applied  ? 

Art.  113. — When  will  one  fluid  float  upon  another? 

Art.  114. — What  is  stated  with  regard  to  a  body  specifically  lighter 
than  a  fluid?  What  illustration  is  given  of  this?  How  do  the 
specific  gravities  of  water  and  cork  compare  with  each  other  ?  | 
Upon  what  principle  is  it  that  boats,  ships,  &-c.,  are  made  to 
float  upon  the  water?  What  rules  have  been  formed  from  the 
knowledge  of  the  specific  gravity  of  water  and  the  materials  of 
which  vessels  are  composed  ? 

Art.  115. — What  standard  has  been  adopted  to  estimate  the  specific 
gravity  of  substances  in  general  ?  Why  could  not  metals  have 
been  adopted?  Why  is  distilled  water  used?  What  bodies  will 
sink  when  immersed  in  water?  What  will  float?  What  is 
the  weight  of  a  cubic  foot  of  water?  What  is  the  use  of  the 
table  ?  How  does  the  specific  gravity  of  living  men  compare 
with  that  of  water?  Which  is  the  greater,  the  specific  grav- 
ity of  sea  water,  or  of  lakes  and  rivers  ?    Why  ? 

Art.  116. — How  is  the  specific  gravity  of  bodies  that  will  sink  in  water 
ascertained?    What  illustration  is  given? 

Fig.  58.  Explain  Fig.  58."  Why  will  gold  weigh  less  in  the  water  than 
out  of  it?  How  does  this  upward  pressure  of  the  particles  com- 
pare with  the  downward  pressure  of  a  quantity  of  water  of  the 
same  dimensions  ?  What  follows  from  this?  What  rule  is  given 
with  regard  to  all  bodies  heavier  than  water  that  are  immersed 
in  it?  What  is  the  specific  gravity  of  a  body?  What  is  the 
reason  that  a  bucket  of  water,  drawn  from  a  well,  is  heavier 
when  it  rises  above  the  surface  of  the  water,  than  while  it  is 
below  it  ? 

Art.  1 1  T. — How  can  the  specific  gravity  of  a  body  that  will  not  sink  in 
water  be  ascertained  ?  What  illustration  is  given?  By  whom 
was  the  method  of  ascertaining  the  specific  gravities  of  bodies 
discovered?    In  what  manner  di-d  he  ascertain  it? 

Art.  118. — What  is  an  hydrometer?  Upon  what  principle  is  it  con- 
structed? Explain  its  construction.  In  what  proportion  does 
the  scale  sink? 


QUESTIONS  FOR  EXAMINATION. 


829 


HYDUAULICS. 

Art.  1 19.— Of  what  does  Hydraulics  treat?  What  retards  the  motion 
of  water?  Why  does  the  surface  of  a  canal  or  river  have  a 
greater  velocity  than  any  other  part? 

Art.  1 20. — Does  the  fulness  of  a  vessel,  from  the  orifice  of  which  a  fluid 
is  running,  have  any  effect  upon  its  velocity  ? 


Fig.  56. 


Fig.  58. 


330 


aUESTIOXS  FOR  EXAMIXATIOX. 


Art.  121» — When  a  fluid  spouts  from  several  orifices  in  the  side  of  a 
vessel,  from  which  is  it  thrown  to  the  greatest  distance? 

Art.  122> — What  effect  will  a  pipe,  fitted  to  an  orifice,  have  with  regard 
to  the  quantity  discharged?  What  will  be  the  effect  if  the  pipe 
project  into  the  vessel  ?  How  can  the  quantity  discharged 
through  a  pipe  or  orifice,  be  increased?  Why  will  heat  in- 
crease it  ? 

Art.  123* — How  can  the  velocity  of  a  current  of  water  be  ascertained? 
Fig.  59.     What  does  Fig.  59  represent,?  Explain  it.   How  is  the  rapidity 
of  the  current  estimated?   What  is  the  use  of  the  instrument? 

Art.  124r. — What  causes  waves?  What  is  sometimes  done  to  remove 
this  friction? 

Art.  135. — What  instruments  are  used  for  raising  liquids? 

Art.  126. — Where  is  the  chain-pump  used?  What  figure  represents  it ? 
Fig.  60.     Explain  the  figure. 

Art.  127  • — What  is  said  of  the  screw  of  Archimedes? 
Fig.  61.     Explain  the  use  of  the  screw  by  Fig.  61. 

Art.  128, — How  are  springs  and  rivulets  formed? 
Fig.  62.     Explain  Fig.  62. 

Art.  129. — How  high  will  a  spring  rise? 

Art.  130. — How  are  fountains  formed?  How  high  will  the  waterspout 
through  the  ducts  ?  What  prevents  the  fluid  from  rising  to  the 
same  height  vWth  the  reservoir  ? 

Fig.  63.     Explain  the  method  of  making  artificial  reservoirs  by  Fig.  63. 

Art.  131. — What  is  the  siphon? 

Fig.  64.  What  figure  represents  a  siphon?  Explain  it  In  what  man- 
ner is  the  siphon  used  ?  How  can  the  siphon  be  used  to  show 
the  equilibrium  of  fluids  ?  How  high  will  the  liquid  rise  in  each 
side  of  the  siphon  ?    What  is  Tantalus'  cup  ? 

Fig.  65.     What  does  Fig.  65  represent?    Explain  it. 

Art.  13  2. — How,  and  for  what  purposes  is  water  used  as  a  mechanical 
agent  ?  How  many  kinds  of  w^ater-wheels  are  there  ?  What 
are  they? 

Art.  133. — What  is  the  overshot  wheel?  Where  does  it  receive  its 
motion  ? 

Fig.  66.  Explain  Fig.  66.  What  causes  the  wheel  to  turn?  How  does 
this  wheel  compare  in  power  with  the  other  water-wheels  1 

xVrt.  134. — What  is  the  undershot  wheel  ?  Where  does  it  receive  its 
motion  ? 


332 


aUESTIONS   FOR  EXAMINATION. 


Fig.  67.  What  does  Fig.  67  represent?  How  does  this  wheel  differ  from 
the  overshot  ? 

Art.  135. — What  is  the  breast- wheel ?    How  is  it  set  in  motion? 

Fig.  68.  What  figure  represents  the  breast-wheel?  Explain  it.  To 
what  is  the  motion  given  to  the  wheels  which  have  been  de- 
scribed, communicated  ? 

PNEUMATICS. 
Art.  136. — Of  what  does  Pneumatics  treat? 

Art.  13  T. — WTiat  is  the  air  which  we  breathe  ?  How  far  does  it  extend 
above  the  surface  of  the  earth  ?  Does  it  possess  properties  com- 
mon to  liquids  in  general?  How  does  its  specific  gravity  com- 
pare with  that  of  water?  Of  what  two  principal  ingredients 
does  the  air  consist  ?  What  is  the  proportion  of  these  parts  to 
each  other  ? 

Art.  138. — What  other  fluids  are  named  belonging  to  the  class  of  elas- 
tic fluids? 

Art.  139. — Have  the  air  and  other  similar  fluids  weight?  With  what 
power  alone  has  heat  to  contend  in  aeriform  fluids  ? 

Art.  140. — ^Vhat  two  principal  properties  has  the  air? 

Art.  14:1. — What  is  the  weight  of  a  column  of  air  one  inch  square  at 
the  base,  and  reaching  to  the  top  of  the  atmosphere?  Is  the 
pressure  exerted  equally  in  all  directions? 

Art.  14:2. — What  is  meant  by  the  elasticity  of  the  air?  How  do  the 
aeriform  fluids  differ  from  liquids  ?  When  is  the  air  said  to  be 
rarefied  ?  When  condensed  ?  Is  the  air  near  the  surface  of 
the  earth  rare  or  dense  ? 

Art.  143. — How  does  the  air  become  a  mechanical  agent? 

Art.  14:4:. — What  is  a  vacuum? 

Art.  14:5. — What  is  a  barometer?  What  does  the  word  barometer 
mean?  What  is  a  thermometer?  What  does  the  word  ther- 
mometer mean?  What  is  a  hygrometer?  What  does  the 
word  hygrometer  mean  ? 

Fig.  69.     What  figure  represents  a  barometer?    Explain  its  construction. 

What  height  of  mercury  is  the  pressure  of  the  atmosphere  ca- 
pable of  sustaining  ?  What  effect  has  the  pressure  of  the  at- 
mosphere on  the  mercury  in  the  tube?  In  what  proportion 
does  the  mercury  rise  and  fall  ?  In  what  way  can  barometers 
be  made  of  other  fluids?  Why  is  mercury  used  in  preference 
to  any  other  fluid?  Is  the  air  heaviest  in  wet  or  dry  weather,? 
On  what  principle  is  the  pressure  of  the  atmosphere  on  the  mer- 


aUESTIONS   FOR  EXAMINATION. 


333 


ciiry,  ill  the  cup  of  a  barometer,  exerted?  What  follows  from 
this?  For  what  other  purpose,  besides  measuring  the  pressure 
of  the  atmosphere,  and  foretelling  the  variations  of  the  weather, 
is  the  barometer  used?  Is  the  air  the  more  dense  at  the  sur- 
face of  the  earth  or  upon  a  hill?  What  is  a  thermometer? 
Fig.  70  What  figure  represents  a  thermometer?  Explain  its  construc- 
tion. What  effect  have  heat  and  cold  on  most  substances? 
What  follows  from  this?  Whose  scale  is  generally  used  in  this 
country  ?  For  what  is  the  hygrometer  used  ?  Of  what  kind 
of  substances  may  it  be  constructed?  What  experiment  is 
given  in  the  note  to  show  the  quantity  of  moisture  raised  from 
the  ground  by  the  heat  of  the  sun  ? 


334  auESTioNS  for  examination. 


Art.  146. — Is  air  impenetrable,  like  other  substances?  How  is  this 
shown  ?    Upon  what  principle  is  the  diving-bell  constructed  ? 

Fig.  71.  What  figure  represents  the  diving-bell?  Why  does  not  the 
water  rise  in  the  bell?    Explain  the  figure. 


Art.  14T,^By  what  means  is  water  raised  in  the  common  pump?  How 
is  the  pressure  removed  ? 

Fig.  72.  What  figure  represents  the  common  pump?  Explain  it.  Which 
of  the  mechanical  powers  is  the  handle  of  the  pump?  How 
high  can  water  be  raised  by  the  common  pump?  Why?  Why 
is  the  common  pump  sometimes  called  the  lifting-pump  ? 


Art.  148. — How  does  the  forcing-pump  differ  from  the  common  pum.p? 
Fig.  73.     What  figure  represents  the  forcing-pump  ?    Explain  it. 


Art.  14:9. — What  is  wind?  In  what  two  ways  may  the  motion  of  the 
air  be  explained  ?  Explain  the  manner  in  which  the  air  is  put 
in  motion.  How  are  the  north,  south,  east,  and  west  winds 
produced? 


Art.  150. — What  is  an  air-pump?  What  is  the  vessel  called  from 
which  the  air  is  exhausted?  On  what  principle  are  all  air- 
pumps  constructed?    Can  a  perfect  vacuum  ever  be  obtained? 

Fig.  74.     Describe  the  air-pump  represented  by  Fig.  74. 

Fig.  75.  Describe  Wightman's  patent  lever  air-pump,  represented  by 
Fig.  75. 


Art.  151. — Experiments  with  the  air-pump. 

Fig.  76.     What  does  Fig.  76  represent?    Explain  the  experiments  that 

are  made  with  them. 
Fig.  77.     What  does  Fig.  77  represent?    Explain  the  experiments  made 

with  it. 


33G 


QUESTIONS    FOR  EXAMINATION. 


Fio-.  7S.     What  does  Fig.  78  represent  ?    Explain  the  experiments  made 

with  it. 

Fio-.  79.  What  does  Fig.  79  represent?  Explain  the  experiments  made 
with  it. 

Fig.  SO.     What  does  Fig.  80  represent  ?    Explain  its  use. 

Fig.  81.     What  does  Fig.  81  represent,  and  what  experiments  are  made 

with  it  ? 

Fig.  8i^.     What  does  Fig.  8:3  represent  ?    Explain  its  nse. 
Fig.  83.     What  does  Fig.  83  represent  ?    Explain  the  experiment  made 
with  it. 

Fis;.  84.     What  does  Fig.  84  represent  ?    Explain  its  use. 

Fio;.  85,  86.    Explain  the  uses  of  Figures  85  and  86. 

Fig.  87.     What  does  Fig.  87  represent  ?    Explain  its  use. 

Fig.  88,,  89.    Explain  the  Figures  88  and  89,  and  the  experiments  which 

are  made  with  them. 
Fig.  90.     What  does  Fig.  90  represent?   Explain  the  experiments  which 

may  be  made  with  it. 
Ficr.  91.     What  does  Fig.  91  represent  ?    Explain  the  experiments  to  be 

made  with  it. 

ACOUSTICS. 

Art  15  2.— Wliat  is  that  science  called  which  treats  of  the  nature  and 
laws  of  sound  ?    What  does  it  include  ? 

Art-  153.— What  causes  sound  ?  What  illustrations  are  given  to  prove 
this? 

Art.  154. — In  what  proportion  are  sounds  loud  or  faint?  Wiy  does  a 
bell  sound  louder  in  cold  than  in  warm  weather  ?  Why  is 
sound  fainter  on  the  top  of  a  mountain  than  near  the  surface 
of  the  earth? 

Art.  15  5. — What  are  sonorous  bodies? 

Art.  156. — To  what  do  sonorous  bodies  owe  their  sonorous  property  ? 
Are  all  elastic  bodies  sonorous  ? 

Art.  15T. — What  causes  the  sound  produced  by  a  musical  string? 

Upon  what  does  the  height  and  depth  of  the  tone  depend? 
Which  strings,  iu  a  musical  instrument,  produce  the  low 
tones?  Why? 


33S 


aUESTIOXS   FOR  EXAMINATIOX. 


Fig.  92.     Explain  Fig.  92. 

Art.  1o8. — Upon  what  is  the 'science  of  harmony  founded?  How  is 
the  chord  of  an  octave  produced?  How  is  the  chord  of  a 
fifth  produced?  How  is  a  musical  chord  produced?  A  dis- 
cord? 

Art.  15  9. — Upon  what  does  the  quality  of  the  sound  produced  by  strings 
depend  ?  Upon  what  does  tiiat  produced  by  wind  instruments 
depend?  What  strings  produce  the  lowest  tones ?  How  may- 
different  tones  be  produced  from  the  same  string  ?  How  may 
different  tones  be  produced  from  the  same  wind  instrument? 

Art.  lf»0.— What,  in  some  degree,  affects  the  quality  of  the  sound  of 
ail  musical  instruments?  What  effect  have  heat  and  cold  on 
the  materials  of  which  the  instrument  is  made  ?  What  follows 
from  this  ?  Why  are  most  musical  instruments  higher  in  tone, 
or  sharper,  in  cold  weather? 

Art.  161. — Through  which  is  sound  communicated  more  rapidly,  and 
with  greater  power,  through  solid  bodies,  or  the  air?  How  fast 
is  it  conducted  by  water  ?  How  fast  by  solids  ?  What  exam- 
ples are  given  to  show  that  sound  is  communicated  more  rap- 
idly through  sohd  bodies  than  the  air  or  fluids  ? 

Art.  162.— What  is  ^  stethescope  ?  Of  what  does  it  consist?  For 
what  is  it  used'? 

Art.  163.  How  fast  does  sound  move  ?    Does  the  force  or  direction  of 

the  wind  make  any  difference  in  its  velocity?  What  advan- 
tage results  from  this  uniform  velocity  of  sound?  How  can 
the  distance  of  a  thunder-cloud  be  ascertained? 

Art  le-l.— How  is  an  echo  produced?  VThy  cannot  an  echo  be  heard 
at  sea,  or  on  an  extensive  plain  ?  How  must  a  person  stand  in 
order  to  hear  an  echo?  By  what  law  is  sound  communicated 
and  reflected  ?  What  anecdote  is  related  of  Dionysius  ?  Upon 
what  principle  are  speaking-trumpets  constructed?  Explain 
the  manner  in  which  the  vibrations  of  the  air  are  reflected. 
Upon  what  principle  are  hearing-trumpets  constructed  ?  How 
far  does  the  musical  instrument,  called  the  trumpet,  act  upon 
the  principle  of  the  speaking-trum.pet  ?  How  can  the  continued 
sound,  given  by  some  shells,  when  held  near  the  ear,  be  ex- 
plaineil  ? 

Art.  165.— Upon  what  principle  may  whispering-galleries  be  con- 
stmcted  ? 

Art.  166.— In  what  way  can  sounds  be  couveyed  to  a  much  greater 
distance  than  through  the  air  ?  What  are  the  tubes,  used  to 
convey  sounds,  called  ?    Why  do  the  softer  kinds  of  furniture 


QUESTIONS   FOR  EXAMINATION. 


339 


in  a  room  affect  the  quality  of  the  sound?  What  general  rule 
is  given  with  regard  to  the  reflection  of  sound?  Is  the  air  a 
better  conductor  when  it  is  humid,  or  when  it  is  dry  ?  Why 
can  a  sound  be  heard  better  in  the  night  than  in  the  day  ? 

Art.  16T. — How  is  the  sound  of  the  human  voice  produced?  How 
are  the  tones  varied  and  regulated  ?  Upon  what  does  the 
management  of  the  voice  depend?  What  is  ventriloquism? 
Was  this  art  known  to  the  ancients?  Whht  is  supposed,  by 
some  authors,  concerning  the  responses  at  Delphi,  Ephesus, 
&c.  ?    Is  ventriloquism  a  natural  gift,  or  an  acquired  one  ? 


PYRONOMICS. 

Art.  168. — What  is  Pyronomics?  What  is  said  in  regard  to  the  na- 
ture of  heat?  Is  it  ponderable  or  imponderable?  What  ef- 
fect has  heat  upon  bodies?  What  two  forces  continually  act 
in  opposition  to  each  other?  In  what  can  the  effect  of  heat 
be  seen  ?  How  does  it  separate  the  particles  ?  What  would 
be  the  effect  were  the  heat  removed  ?  Upon  what  has  heat 
the  most  remarkable  effect?  How  does  it  affect  it?  What 
effect  has  heat  upon  air?  How  is  rain  produced?  What  is 
stated  with  regard  to  heat  ?  Can  the  progress  of  heat  be  ar- 
rested? What  is  caloric  ?  In  what  two  states  does  heat  exist? 
What  is  free  heat  ?  Give  some  examples  of  free  heat.  What 
is  latent  heat  ?  Give  some  examples  of  latent  heat.  How  are 
the  terms  heat  and  cold  generally  used  ?  V/hat  illustration  of 
this  is  given  ? 

Art.  169.— What  are  the  three  principal  effects  of  heat  on  bodies  to 
which  it  is  applied?  Give  an  example  of  each.  What  are  the 
sources  of  heat  ? 

Art.  ITO. — In  what  way  does  heat  tend  to  diffuse  itself?  Why  do  bodies 
of  the  same  absolute  temperature  appear  to  possess  different  de- 
grees of  heat  ?  What  illustration  of  this  is  given  ?  What  ap- 
pears from  this? 


Fis-  92. 


340 


aUESTIONS    FOR  EXAMINATION. 


Atr.  ITl.— What  causes  the  difference  in  the  warmth  of  substances 
used  for  clothing? 

Art.  it 2.— In  what  two  ways  is  heat  propagated?  When  is  it  propa- 
gated by  conduction?    When  is  it  propagated  by  radiation  ? 

Art.  it  3. —Do  all  bodies  conduct  heat  with  the  same  degree  of  facility? 

What  bodies  are  the  best  conductors?  In  what  order  do  the 
metals  stand  with  respect  to  their  conducting  power  ?  Is  wood 
a  good  conductor  of  heat?  Why  are  wool,  fur,  &c.,  so  effica- 
cious in  preserving  the  warmth  of  the  body?  What  is  re- 
lated in  the  note  with  regard  to  the  conducting  power  of 
heat? 

Art.  1T4.— What  bodies  reflect  the  heat?  What  bodies  absorb  the 
heat?  Why  do  bright  bodies,  when  placed  near  the  fire,  sel- 
dom become  heated  ?  Will  snow  melt  most  readily  under  white 
or  black  cloth  ? 

Art.  it 5.— What  effect  is  produced  on  all  bodies  when  violently  com- 
pressed or  extended  ?  What  experiments  are  here  related  to 
illustrate  this?  What  is  said  of  the  air  when  strongly  com- 
pressed ? 

Art.  1T6.— Into  what  classes  are  all  substances,  as  affected  by  heat, 
divided  ?  What  substances  are  combustible  ?  What  substances 
are  incombustible  ? 

Art.  ITT  .—What  is  a  pyrometer  ?  Of  what  does  it  consist  ?  How  does 
Wedgewood's  pyrometer  measure  high  temperatures  ? 

Art.  it 8.— What  is  the  most  obvious  and  direct  effect  of  heat  on  a 
body?  What  application  of  this  principle  is  related  in  the 
note?  What  is  said  of  the  effect  of  heat  and  cold  on  glass? 
When  hot  water  is  suddenly  poured  into  a  cold  glass,  why  will 
the  glass  crack?  When  cold  water  is  applied  to  a  heated 
glass,  why  will  the  glass  crack? 

Art.  1T9.— Is  the  expansion  caused  by  heat  in  solid  and  liquid  bodies 
the  same  in  all  substances  ?  How  do  aeriform  fluids  differ,  in 
this  respect,  from  solid  and  liquid  bodies?  Upon  what  does 
the  expansion  of  solid  bodies  in  some  degree  depend?  Why 
has  heat  more  power  over  gases  and  vapors  ? 

Abt.  180.— What  effect  has  heat  and  cold  upon  the  density  of  all  sub- 
stances? What  exception  is  there  to  this  remark  ?  Why  are 
the  vessels,  containing  water  and  other  similar  fluids,  so  often 
broken  when  the  liquid  freezes  in  them?  Why  does  ice  float 
upon  the  water,  instead  of  sinking  in  it  ?  What  is  stated  in  the 
note  with  regard  to  this  property  of  water  ? 


auEsnoNs  for  examination. 


341 


Art.  181. — Ciiu  all  bodies  bo  raised  to  the  same  temperature  by  the 
same  quantities  of  heat?  What  bodies  retain  their  heat  the 
longest  ? 

Art.  182. — What  becomes  of  the  heat  which  is  thrown  upon  a  bright 
or  polished  surface  ?  How  do  the  angles  of  incidence  and  re- 
flection compare  with  each  other? 

Art.  183. — When  is  water  converted  into  steam  or  vapor? 

Art.  184:. — How  does  the  temperature  of  the  steam  compare  with  that 
of  the  liquid  from  which  it  is  formed  while  it  remains  in  contact 
with  that  liquid  ? 

Art.  185. — By  what  is  the  elasticity  of  steam  increased  and  diminished? 

Upon  what  does  the  amount  of  pressure,  which  steam  exerts, 
depend  ? 

Art.  1 86. — What  is  the  great  and  peculiar  property  of  steam,  on  which 

its  mechanical  agencies  depend? 
Art.  18T.— What  is  the  steam-engine? 

Art.  188. — How  much  larger  space  does  steam  occupy  than  water?  By 
what  mode  is  steam  made  to  act?  By  what  impulse  does  the 
piston  rise  ?  What  causes  the  piston  to  descend  ?  What  im- 
provement did  Mr.  Watt  introduce  into  the  steam-engine  ? 

Fig.  93.     What  does  Fig.  93  represent?    What  are  the  principal  parts? 

What  does  A  represent  ?  What  does  C  represent  ?  What  does 
B  represent  ?  What  does  K  represent  ?  By  what  is  the  steam 
led  off  from  the  cylinder?  What  does  L  represent?  What 
does  O  represent?  What  does  M  represent?  What  does  N 
represent  ?  What  does  R  represent?  When  the  valves  are  all 
open,  what  becomes  of  the  steam?  When  the  valves  F  and  Q 
are  closed,  and  G  and  P  open,  upon  what  does  the  steam  press? 
What  does  the  cylinder  draw  with  it  in  its  descent  ?  Which  of 
the  mechanical  powers  is  this  working-beam  ?  What  are  at- 
tached to  this  working-beam?  What  is  their  use?  What  be- 
comes of  the  steam  when  the  stop-cocks  G  and  P  are  closed,  and 
Fig.  93. 


342 


aUESTIONS  FOR  EXAMINATION. 


F  and  Q  are  open?  Hovv^  is  the  regular  and  continued  motion 
produced?  To  what  is  this  motion  of  the  piston  communica- 
ted ?  What  is  the  use  of  the  air-pump  M  ?  For  what  is  the 
safety-valve  R  used  ? 

Art.  189.— How  is  the  power  of  a  steam-engine  expressed?  What  is 
an  engine  of  100  horse  power? 

Art  190.— What  are  the  principal  forms  in  which  the  steam-engine  is 
constructed?  How  do  they  differ  from  each  other  ?  What  be- 
comes of  the  steam  after  having  moved  the  piston  in  the  non- 
condensing  engines?  What  kind  of  engines  is  generally  used 
on  railroads?  What  becomes  of  the  steam  after  having  moved 
the  piston  in  the  condensing  engines? 

Fig  94.  What  does  Fig.  94  represent?  What  does  A  represent ?  What 
does  B  represent?  What  does  C  represent?  What  does  D 
represent?  What  does  E  represent?  What  does  F  represent  ? 
What  does  GG  represent?  What  does  I  represent?  What 
does  K  represent?  What  does  LL  represent?  What  does 
M  M  represent  ?  What  does  N  N  represent  ?  What  does  O  O 
represent?    W^hat  is  said  of  the  governor  ? 


Fig.  94. 


344 


aUESTIONS   FOR  EXAMINATION". 


OPTICS. 

Art.  192c— Of  what  does  Optics  treat?  Into  what  classes  does  the 
science  of  Optics  divide  all  substances  ?  What  are  luminous 
bodies?  Give  an  example  of  a  luminous  body.  What  are 
transparent  bodies  ?  Give  an  example  of  a  transparent  body. 
What  are  translucent  bodies?  Give  an  example  of  a  translu- 
cent body.  What  are  reflecting  substances?  Give  an  exam- 
ple of  a  reflecting  body.  What  are  refracting  substances? 
What  are  opaque  substances?  What  is  light?  What  did  Sir 
Isaac  Newton  suppose  it  to  be  ?  What  other  opinions  have 
been  formed  concerning  it  ? 

Art.  193. — What  is  a  ray  of  light? 

Art.  194. — When  are  rays  of  hght  said  to  diverge? 

Fig.  96.     What  does  Fig.  96  represent? 

Art.  195. — When  are  rays  of  light  called  converging?    What  is  the 

point,  at  which  converging  rays  meet,  called? 
Fig.  97.  What  does  Fig.  97  represent  ? 
Art.  196. — What  is  a  beam  of  Hght? 
Fig.  98.  What  does  Fig.  98  represent  ? 
Art.  19T.— What  is  a  pencil  of  hght? 
Art.  198. — What  is  a  medium 

Art.  199. — In  what  manner  do  the  rays  of  light  proceed  from  terrestrial 
bodies  ?  In  what  kind  of  hues  do  the  rays  of  light  proceed  from 
the  sun? 

Art.  200. — In  what  way  is  light  projected  forward  from  any  luminous 

body  ?    With  what  rapidity  does  it  move  ? 
Art.  201. — From  what  point,  in  a  luminous  body,  does  light  radiate? 
Art.   How  is  a  shadow  produced?    Why  are  shadows  of  di{Ferent 

degrees  of  darkness?    To  what  is  the  darkness  of  a  shadow 

proportioned,  when  the  shadow  is  produced  by  the  interruption 

of  the  rays  from  a  single  luminous  body  ? 
Art.  203.  What  is  said  of  the  shadow  of  the  opaque  body  when  the 

luminous  body  is  the  larger  ? 
Fig.  99.     Explain  Fig.  99. 

Art.  204.— What  is  said  of  the  shadow  of  the  opaque  body  when  the 

luminous  body  is  the  smaller  ? 
Fig.  100.   Explain  Fig.  100. 

Art.  2 O 5.— How  many  shadows  are  produced  when  several  luminous 

bodies  shine  upon  the  same  object 
Fig.  101.   Explain  Fig.  101. 


aUKSTTONS    FOR  EXAMINATION. 


345 


Art.  JiOO. — What  is  tho  consequence  wlien  rays  of  light  fall  upon  an 
opaque  body  which  they  cannot  pass?  What  is  meant  by  the 
reflection  of  light? 

Art.  20T« — What  is  Catoptrics? 

Art.  208. — By  what  laws  is  light  governed  ?  How  is  light  reflected 
when  it  falls  perpendicularly  on  an  opaque  body?  How  is  it 
reflected  when  it  falls  obliquely?  How  do  the  angles  of  inci- 
dence and  reflection  compare  with  each  other  ?  By  what  light 
are  opaque  objects  seen  ?  By  what  light  are  luminous  objects 
seen  ? 

Fig  102.  Explain  the  angles  of  incidence  and  reflection.  Is  the  same 
principle  applicable  to  all  kinds  of  surfaces?  Explain  its  appli- 
cation to  mirrors. 


Fig.  96.  Fig.  97. 


Fig.  101.  Fig.  102 


15* 


346  auESTioxs  foe  examination. 

Art.  209.-Why  is  the  intensity  of  light  diminished  every  time  it  is  re- 
flected ? 

Art  210.-Does  every  portion  of  a  refiecting  surface  reflect  an  entire 
ima.e  of  the  luminons  body  shining  upon  it  ?  When  the  sun  or 
moon  shines  upon  a  sheet  of  water,  why  do  we  not  see  an  im- 
a^e  reflected  from  every  portion  of  the  surface  ?  Why  do  ob- 
jects, seen  by  moonlight,  appear  fainter  than  when  seen  by 
daylight!  By  what  light  does  the  moon  shine?  What  absorbs 
some  of  the  rays  of  light  in  traversing  the  atmosphere  ? 

Art.  311.-H0W  are  all  objects  seen?  Why  can  none  but  luminous 
bodies  be  seen  in  the  dark? 

Art  312.-What  kind  oT  an  image  is  formed  when  rays  of  light,  pro- 
ceeding from  an  object,  enter  a  small  aperture  1 

Fig.  103.  Illustrate  this  by  Fig.  103.  What  is  a  camera  obscura  ?  How 
can  a  portable  camera  obscura  be  made  ? 

Art.  313.— How  is  the  angle  of  vision  formed? 

Fio-  104.  Explain  Fig.  104. 

i-rg.   Oo.  What  does  Fig.  105  represent?    ^hat  effect  has  the  ne.mess 
^  of  the  object  to  the  eye,  on  the  angle  ?    Illustrate  th,s  by  the 

figure.  Upon  what  does  the  apparent  size  of  an  object  depend? 
Why  do  objects  appear  so  large  ?    To  what  is  the  art  of  per- 
spective drawing  indebted  for  its  accuracy? 
Am.  21t.-How  large  an  angle  must  a  body  subtend  to  be  visible  ? 

215— When  is  the  motion  of  a  body  invisible?  Why  is  the  mo- 
tion of  the  heavenly  bodies  invisible  ?  Upon  what  does  the  real 
velocity  of  a  body,  in  motion  round  a  pomt,  depend  ? 
Fig.  106.  Explain  Fig.  106.  '  Why  does  the  velocity  of  both,  to  an  eye  at 
F,  appear  to  be  the  same? 
2I6.-H0W  many  kinds  of  mirrors  are  there?  What  are  plain 
mirrors?  Do  they  magnify  or  diminish  the  object ?  What  are 
convex  mirrors?  What  part  of  a  sphere  is  a  convex  mirror? 
What  are  concave  mirrors?  What  part  of  a  sphere  is  a  con- 
cave  mirror? 

Via  107    lu  Fig.  107,  which  part  of  the  sphere  represents  a  convex  m.r- 
°  ror?    Which  part  a  concave  mirror?    Explain  the  figure. 


Which  part  a  concave  mirror 
Art.  317.-HOW  does  the  image  of  an  object  reflected  from  a  convex 

mirror  compare  with  the  object? 
Fig.  108.  Give  the  illustration. 

Art.  218  What  is  the  focus  of  a  concave  mirror? 

Art'  319.-When  an  object  is  further  from  the  concave  mirror  than  the 
focus,  how  will  the  image  appear  ?  When  the  object  is  between 


aUESTIONS   FOR  EXAMINATION. 


347 


the  mirror  and  the  focus?  How  must  the  object  bo  placed  that 
the  image  may  appear  upright?  And  as  the  object  iy  removed 
towards  the  mirror? 

Art.  — If  the  object  bo  placed  between  the  mirror  and  the  focus, 

how  does  the  imago  compare  with  the  object? 


Fig.  103. 


Fig  107. 


H 


Fig.  108. 


Fig.  104. 


Fig.  106. 


0         B  ^ 


p 

—  X 

348  auESTioNS  for  examination. 


Fig.  109.  Explain  Fig.  109.  What  peculiar  properties  have  concave 
mirrors  ?  What  facts  are  stated  with  regard  to  convex  mirrors, 
as  resulting  from  the  fundamental  law  of  Catoptrics  ?  What  is 
said  of  parallel  rays?  What  is  said  of  diverging  rays?  W^hat 
is  said  of  converging  rays,  when  they  tend  towards  the  focus  of 
parallel  rays  ?  What  is  said  of  converging  rays,  wheu  they 
tend  to  a  point  nearer  the  surface  than  the  focus?  What  is 
said  of  converging  rays,  when  they  tend  to  a  point  between  the 
focus  and  the  centre  ?  What  is  said  of  converging  rays,  when 
they  tend  to  a  point  beyond  the  centre  ?  What  is  said  of  con- 
verging rays,  when  they  tend  to  the  centre?  What  is  said 
with  regard  to  parallel  rays,  when  reflected  from  a  concave 
surface  1  What  is  said  of  converging  rays?  What  is  said  of 
diverging  rays,  if  they  diverge  from  a  focus  of  parallel  rays  ? 
What,  if  from  a  point  nearer  to  the  surface  than  that  focus? 
What,  if  from  a  point  between  that  focus  and  the  centre?  If 
from  a  point  beyond  the  centre  ?    If  from  the  centre? 

Art.  221  .—What  is  Dioptrics  ?  ^ 

Art.  222  What  is  meant  by  the  refraction  of  liglW?    When  does  this 

take  place  ? 

Art.  223  What  is  a  medium,  in  Optics?    Give  some  examples  of 

media.    In  what  proportion  is  a  medium  dense  or  rare  ? 

Art.  224:.— What  are  the  three  fundamental  laws  of  Dioptrics? 

Fig.'llO.  Illustrate  the  first  law  by  the  line  A  B,  in  Fig.  110.  Illustrate 
the  second  law  by  the  line  C  B.  Illustrate  the  third  law  by 
the  line  F  B.  In  what  proportion  does  the  refraction  increase 
or  diminish?  Why  does  an  oar  or  a  stick,  when  partly  im- 
mersed in  water,  appear  bent?  Why  does  the  part  which  is  in 
the  water  appear  higher  than  it  really  is?  Why  does  a  body 
of  water,  when  viewed  obliquely,  appear  more  shallow  than  it 
really  is?  In  what  direction  can  we  look  so  as  to  cause  no 
refraction  ?    What  experiment  is  here  related  ? 

Art.  225.— Why  do  we  not  see  the  heavenly  bodies  in  their  real  situa- 
tion ?  In  what  direction  do  we  see  them  ?  What  causes  twi- 
light? Upon  what  does  the  duration  of  twilight  depend? 
What  other  reason  is  given  why  we  do  not  see  the  heavenly 
bodies  in  their  true  situation?  When  does  the  refraction  of 
light  not  affect  the  appearance  of  the  heavenly  bodies  ?  Why 
do  the  heavens  appear  bright  in  the  daytime  ? 

Art.  226.  What  effect  is  produced  when  a  ray  of  light  passes  from  one 

medium  to  another,  and  through  that  into  the  first  again? 
Why  does  the  refractive  power  of  flat  w^indow-glass  produce 
no  effect  on  objects  seen  through  it? 


QUESTIONS   FOR  EXAMINATION. 


349 


Airr.  2*17, — What  is  a  lens?  How  aro  all  leiisos  to  be  coiisidcrod  ? 
What  is  a  single  convex  lens  ? 

Fig.  111.   What  part  of   Fig.  Ill   represents  a  single  convex  lens? 

What  is  a  single  concave  lens?  What  part  of  Fig.  Ill  repre- 
sents a  single  concave  lens  ?  What  is  a  double  convex  lens  ? 
What  part  of  Fig.  Ill  represents  a  double  convex  lens?  What 
is  a  double  concave  lens?  What  part  of  Fig.  Ill  represents  a 
double  concave  lens?  What  is  a  meniscus?  What  part  of 
Fig.  Ill  represents  a  meniscus?  What  is  the  axis  of  a  lens? 
What  line  in  Fig.  Ill  represents  the  axis  of  all  the  five 
lenses  ? 

Art.  22S» — What  is  stated  with  regard  to  the  form  of  the  lenses?  How 
is  light  refracted  in  passing  from  a  rarer  to  a  denser  medium  ? 
How,  in  passing  from  a  denser  to  a  rarer  ?  What  must  be 
considered  in  estimating  the  effect  of  lenses?  Through  what 
must  a  perpendicular,  to  any  convex  or  concave  surface,  al- 
ways, when  prolonged,  pass?  What  is  stated  with  regard  to 
convex  lenses  ?    What,  with  regard  to  concave  lenses  ? 

Art.  229, —  What  is  the  focal  distance  of  a  lens?  To  what  is  this  equal 
in  a  single  convex  lens  ?  To  what  is  it  equal  in  a  double  con- 
vex lens  ? 

Fig.  109. 


G 


350 


aUESTIONS   FOR  EXAMINATION. 


Art.  — When  parallel  rays  fall  on  a  convex  lens,  which  one  is  per- 

pendicular to  its  surface?  How  are  the  other  rays,  falling 
obliquely,  refracted  ?  What  property  of  a  convex  lens  gives  it 
its  power  as  a  burning-glass?  Where  are  all  the  parallel  rays 
of  the  sun,  which  pass  through  the  glass,  collected?  How 
does  the  heat  at  the  focus  compare  with  the  common  heat  of 
the  sun?  What  is  related  with  regard  to  the  effects  of  lenses 
produced  by  burning-glasses? 

Art.  231. — What  is  the  first  effect  related  as  resulting  from  the  laws  of 
refraction  with  regard  to  convex  surfaces  ?  W^hat  is  said  of 
diverging  rays?  What  is  said  of  rays  converging  towards  the 
centre  of  convexity?  What  of  rays  converging  to  a  point  be- 
yond the  centre  of  convexity  ?  W^hat  of  rays  converging  to  a 
point  nearer  the  surface  than  the  centre  of  convexity  ?  When 
the  rays  proceed  out  of  a  denser  into  a  rarer  medium,  what 
occurs  ?  What  is  stated  of  parallel  rays,  proceeding  from  a 
rarer  into  a  denser  medium,  through  a  concave  surface?  What 
is  said  of  diverging  rays  ?  What  is  said  of  converging  rays  ? 
Of  what  are  the  above  eight  principles  the  necessary  conse- 
quence ?  What  is  the  reason  that  so  many  different  principles 
are  produced  by  the  operation  of  these  laws? 

Art.  232* — For  what  are  double  convex  and  concave  glasses,  or  lenses, 
used  in  spectacles  ?  What  glasses  are  used  when  the  eye  is 
too  flat  ?    What  are  used  when  the  eye  is  too  round  ? 

Art.  23  3* — Of  what  is  the  eye  composed?  What  are  the  different 
parts  of  the  eye?  First?  Second?  Third?  Fourth?  Fifth? 
Sixth?    Seventh?    Eighth?    Ninth?  Tenth? 

Fig.  112.  What  does  Fig.  112  represent?    Explain  the  figure. 

Fig.  113.  What  does  Fig.  113  represent?  Explain  the  figure.  What 
part  of  the  eye  does  the  cornea  form  ?  Is  its  degree  of  con- 
vexity the  same  in  all  persons  and  all  periods  of  life  ?  What 
is  its  principal  office  ?  From  what  does  the  iris  take  its  name  ? 
What  is  the  use  of  the  iris  ?  'What  is  the  pupil  ?  What  is  its 
form  in  the  human  eye  ?  How  much  more  light  is  the  pupil 
capable  of  admitting,  when  expanded  to  its  utmost  extent,  than 
when  most  contracted  ?  What  is  said  of  those  animals  which 
are  said  to  see  in  the  dark?  What  light,  only,  is  of  use  in 
vision  ?  What  becomes  of  the  light  which  falls  on  the  iris  ? 
What  is  the  aqueous  humor?  W^hat  is  its  form  ?  Of  what  use 
is  it?  What  is  the  crystalline  lens?  What  is  its  office  ?  What 
is  the  vitreous  humor?  Why  do  persons  sometimes  experience 
pain  when  passing  from  a  dark  place  into  strong  light  ?  What 
is  the  shape  of  the  vitreous  humor  ? 


aUESTIONS   FOR   EXAMINATION.  ^351 


Fijr.  114.   Explain  Fijr.  114.    What  is  tlio  retina?    What  is  the  choroid? 

IJy  what  is  its  outer  and  inner  surface  covered  ?  Wiiat  is  its 
office  ?  What  is  the  opinion  of  some  philosophers  with  rejrard 
to  the  choroid?  Wliat  is  the  sclerotica?  From  what  docs  it 
derive  its  name  ?  What  is  its  office  ?  What  are  attached  to 
the  sclerotica  ?    What  is  the  optic  nerve  ? 

Art.  234. — What  philosophical  instrument  does  the  eye  resemble  in  its 
construction  ? 

Fig.  115.  Explain  Fig.  115.  Why  do  the  objects  appear  erect  when  the 
images  are  inverted?  Why  do  we  see  only  one  image  when 
an  image  is  formed  on  both  eyes?  What  are  the  defects  which 
are  remedied  by  the  use  of  concave  and  convex  lenses?  In 
what  other  way  is  the  eye  subject  to  imperfection  ?  Is  there 
any  remedy  for  this  ?  By  what  is  the  convexity  of  the  crys- 
talline humor  increased  or  diminished  ?  What  is  effected  by 
this  means  ? 


Fig.  112.  Fig.  113. 


352 


aUESTIONS   FOR  EXAMINATION. 


Art.  23 5» — What  is  a  single  microscope?    What  is  the  use  of  this  mi- 
croscope 1    What  figure  represents  a  microscope  ? 

Fig.  116.  Explain  Fig.  116.    What , lenses  have  the  greatest  magnifying 
power  ?    What  lenses  have  the  shortest  focus  ? 


Art.  236. — Of  what  does  a  double  microscope  consist?    What  is  the 
use  of  these  two  lenses  ? 

Fig.  117.  What  does  Fig.  117  represent?    Explain  the  figure. 


Art.  23 T. — What  is  the  solar  microscope?    Of  what  does  it  consist? 

By  what,  in  this  microscope,  are  the  sun's  rays  reflected,  and 
upon  what?  For  viewing  what  objects,  only,  is  the  microscope, 
above  described,  used?  How  do  those  microscopes,  used  for 
viewing  opaque  objects,  differ  from  these  ?  How  is  the  image 
then  formed?  How  is  the  magnifying  power  of  a  single  mi- 
croscope ascertained?  Illustrate  this.  How  is  the  magnifying 
of  the  compound  microscope  ascertained  ?  In  what  proportion 
is  the  magnifying  power  of  the  solar  microscope?  Illustrate 
this.  How  may  a  lens  be  made  to  magnify  or  diminish  an 
object? 


Art.  23  8- — What  is  the  magic  lantern?  How  are  objects,  viewed  by 
the  magic  lantern,  generally  represented?  What  figure  repre- 
sents a  magic  lantern? 

Fig.  118.  Explain  Fig.  118.  In  what  proportion  will  the  size  of  the 
image  increase  or  diminish  ? 


Art.  239. — What  is  a  telescope?  How  many  V..  r-  of  telescopes  are 
there  ?  What  are  they  ?  What  is  a  refracting  telescope  1 
What  is  a  reflecting  telescope  ?  Why  is  the  im^age  of  an  ob- 
ject, seen  through  a  refracting  telescope,  less  clear  and  p^rfec^. 
than  when  seen  through  a  reflecting  telescope  ?  How  many 
kinds  of  refracting  telescopes  are  there?  What  are  lluy 
How  do  they  differ  the  one  from  the  other? 

Fig.  119.   What  does  Fig.  119  represent?    Explain  the  figure. 


FiK.  I  in. 


854 


aUESTIONS  FOR  EXAMINATION. 


Fig.  120.  What  does  Fig.  120  represent?    Explain  the  figure. 

Fig.  121.  What  does  Fig.  121  represent?  Explain  the  figure,  are 
mirrors  used  in  reflecting  telescopes  ?  What  is  the  use  of  the 
lens?    What  is  the  advantage  of  the  reflecting  telescope  ? 

Art.  What  is  Chromatics?    What  causes  color? 

Art.  241.— Of  what  is  light  composed?  How  can  these  rays  be  sepa- 
rated ?    By  whom  was  this  discovery  made  ? 

Art.  242. — What  is  a  prism?    How  may  a  prism  be  made? 

Art.  243. — How  many  colors  enter  into  the  composition  of  light?  What 
are  they  ?  Do  these  rays  all  have  the  same  degree  of  refrangi- 
bility  ? 

Art.  244.— What  takes  place  when  light  is  made  to  pass  through  p 
prism  ? 

Fig.  122.  Explain  Fig.  122.  Why  do  the  red  rays  fall  on  the  lowest 
part  of  the  screen  ?  What  is  supposed  with  regard  to  the  red 
rays?  What  with  regard  to  the  blue,  indigo,  and  violet  rays? 
Why  does  the  sun  appear  red  through  a  fog?  Why  does  the 
sky  appear  of  a  blue  color?  What  would  be  the  appearance 
of' the  sky  if  the  atmosphere  did  not  reflect  any  rays?  Is 
white  a  simple  color  ?  How  is  it  produced  ?  The  spectrum 
formed  by  a  prism  being  divided  into  360  parts,  how  many  of 
these  parts  does  the  red  occupy ?  The  orange?  The  yellow? 
The  green?  The  blue?  Tne  indigo?  The  violet?  What 
are  the  colors  of  all  bodies  ?  What  appears  from  the  experi- 
ments of  Dr.  Wollaston  ? 

Art.  245.— How  is  the  rainbow  produced ?  How  is  this  proved ?  First  ? 
Second  ?    Third  ? 

Art.  246.  Upon  what  does  the  color  of  all  bodies  depend?    Of  what 

color  do  bodies  generally  appear  ?  When  will  a  body  appear 
of  a  compound  color?  Of  what  color  will  a  body  appear  that 
reflects  all  the  rays?  When  will  a  body  appear  black?  Is 
color  an  essential  property  of  a  body  ?  Of  what  color  do  bodies 
appear  in  the  dark?  Why  do  some  bodies  appear  diflerently 
by  candlehght?  What  is  necessary  to  produce  color?  What 
experiments  are  related  to  prove  the  truth  of  the  above  ?  What 
rays  does  a  body  reflect  in  the  greatest  abundance  ?  In  what 
proportion  does  it  reflect  the  other  rays?  Why  do  the  green 
leaves  of  a  rose  appear  to  have  a  brown  tinge  ?  What  does 
the  brightness  and  intensity  of  a  color  show?  Why  do  some 
bodies  change  their  color  ? 

Art.  24T.— What  is  a  multiply iug-gl ass?  How  many  times  will  an 
object,  viewed  through  a  multiplying-glass,  be  multiplied? 
What  is  the  principle  of  the  multiplying-glass  ? 


aUESTIONS  FOR  EXAMINATION.  355 


Art.  2l8.^0f  what  does  the  kaleidescope  consist?  From  what  is  the 
word  kaleidescope  derived,  and  what  does  it  mean  ?  By  whom 
was  the  instrument  invented  ?  What  is  here  said  with  regard 
to  the  kaleidescope  ?  In  what  way  does  Optics  treat  of  light  ? 
What  is  the  thermal  property  of  light  ?  Are  the  thermal  prop- 
erties of  the  rays  the  same,  or  different  in  each  color  of  the 
prism  ?  How  do  they  differ  ?  How  do  the  chemical  agencies 
differ?  Do  these  powers  exist  in  all  light,  or  in  solar  light  only? 
Where  is  the  greatest  heating  power  found  ?  Where  the 
greatest  chemical  power  ?  Where  the  greatest  optical  power  ? 
In  what  are  the  chemical  powers  of  light  shown  ?  Name  the 
experiments  and  explain  them.  Explain  the  art  of  Photography, 
or  Heliography.  By  what  name  is  it  now  known  ?  How  else 
are  the  chemical  effects  of  light  seen? 

Fig.  120. 


D  E 


356  auESTioNs  for  examination. 


ELECTRICITY. 

Art.  — What  is  electricity?    How  can  electricity  be  seen?  How 

are  these  effects  exhibited  ?  What  illustration  of  this  is  given  ? 
What  is  said  of  the  surfaces  which  have  acquired  the  power  of 
attraction?  W^hat  are  electrics?  What  are  non-electrics? 
What  is  stated  with  regard  to  the  word  electricity  ?  By  whom 
was  this  property  first  discovered  ?  What  is  stated  with  regard 
to  the  nature  of  electricity?  Whose  and  what  opinion  is 
adopted  in  this  volume  ?  When  is  a  substance  said  to  be 
positively  electrified  ?  When  is  it  said  to  be  negatively  electri- 
fied ?  What  does  positive  electricity  imply  ?  W^hat  does  nega- 
tive electricity  imply?  In  how  many  ways  may  electricity  be 
excited  ?    What  are  the  different  kinds  of  electricity  called  ? 

Art.  25  O. — Into  how  many  kinds  does  the  science  of  electricity  divide 
all  substances  ?    What  are  they  ? 

Art.  251. — What  is  said  with  regard  to  the  communication  of  the  elec- 
tric fluid  from  one  substance  to  another  ?  Will  all  substances 
allow  it  to  pass  through  them  ?  What  bodies  are  called  con- 
ductors ?  What  bodies  are  called  non-conductors  ?  What  has 
been  found,  by  experiment,  with  regard  to  electrics  and  non- 
electrics  ?  What  substances  are  electrics  or  non-conductors  ? 
W^hy  must  these  substances  be  dry  ?  What  substances  are 
non-electrics  or  conductors  ?  What  substances  are  mentioned 
as  imperfect  conductors  ? 

Art.  25  2. — When  is  a  substance  said  to  be  msulated?  Give  the 
examples. 

Art.  253. — When  a  communication  is  made  between  a  conductor  and 
an  excited  surface,  where  is  the  electricity  from  the  excited 
substance  conveyed  ?  When  is  it  said  to  be  charged  ?  When 
a  communication  exists  by  means  of  any  conducting  substance, 
between  a  body  containing  more  than  its  natural  share  of  the 
fluid  and  the  earth,  what  will  become  of  the  redundant  quantity 
which  the  body  possesses  ?  What  illustration  of  this  is  given  ? 
What  follows  if  this  chain  of  conducting  substances  be  inter- 
rupted ? 

Art.  254. — What  is  the  simplest  mode  of  exciting  electricity?  What 
illustration  of  this  is  given  ? 

Art.  255. — What  is  the  electricity  excited  in  glass  called?  What  is 
that  obtained  from  resinous  substances  called  ? 

Art.  256. — What  is  stated  with  regard  to  positive  and  negative  elec- 
tricity ?    What  follows  when  one  side  of  a  metallic,  or  other 


aUESTIONS   FOR  EXAMINATION. 


357 


conductor,  receives  the  electric  fluid  ?  What  follows  when  an 
electric  is  presented  to  an  electrified  body?  What  follows 
when  two  surfaces,  oppositely  electrified,  are  united  ? 

Art.  25  T. — How  do  similar  states  of  electricity  act  on  each  other?  How 
do  dissimilar  states  act  on  each  other?    Give  the  examples. 

Art.  258. — For  what  is  the  Leyden  jar  used? 

Fig.  123.  What  does  Fig.  123  represent?  What  is  a  Leyden  jar?  When 
is  the  Leyden  jar  said  to  be  charged  ?  How  can  the  jar  be  dis- 
charged ?    Can  an  insulated  jar  be  charged  ? 

Art.  259.— Of  what  is  an  electrical  battery  composed?  How  are  the 
inner  coatings  of  the  jars  connected  together?  How  are  the 
outer  coatings  connected?  In  what  way  is  the  battery  charged  ? 

Art.  260.— What  is  the  jointed  discharger  ? 

Fig.  124.  What  does  Fig.  124  represent  ?    Of  what  does  it  consist  ? 

Art.  281, — What  is  necessary  when  a  charge  of  electricity  is  to  be  sent 
through  any  particular  substance  ? 

Art.  262.— In  what  way  do  metallic  rods,  with  sharp  points,  attract  the 
electric  fluid  ?  How  can  the  electricity  be  made  to  pass  off' 
silently  ?  Upon  what  principle  are  lightning-rods  constructed  7 
When  is  electricity  said  to  be  communicated  by  induction  ? 

Art.  263. — When  is  electricity  produced  by  transfer? 

Art.  264.— For  what  purpose  is  the  electrical  machine  constructed  ? 

Upon  what  principle  are  all  electrical  machines  constructed  ? 


Fig.  123. 


Fig.  124. 


358 


aUESTIOXS   FOR  EXAMINATION. 


How  is  the  electricity  excited  ?  Of  what  is  the  amalgam  com- 
posed ?  la  what  form  is  the  glass  surface  made  ?  When  is 
the  machine  called  a  plate  machine  ?  When  is  it  called  a  cyl- 
inder machine  ? 

Fig.  125.  What  does  Fig.  125  represent  ?  Explain  the  figure.  Explain 
the  operation  of  the  machine.  To  what  must  the  chain  be  at- 
tached when  positive  electricity  is  required  ?  To  what  must  it 
be  attached  when  negative  electricity  is  wanted  ?  What  is  the 
first  experiment  mentioned  with  the  electrical  machine  ?  What 
does  the  word  electrometer  mean  ?  Of  what  does  it  sometimes 
consist?  What  is  an  electroscope?  W^hat  is  the  second  ex- 
periment ?  W^hat  is  the  third  ?  What  does  this  show  ?  What 
is  the  fourth  ?    What  is  the  fifth  ? 

Fig.  126.  What  is  the  sixth?  W^hat  is  the  seventh  ?  How  may  the  jar 
be  filled  with  negative  electricity  ?  What  is  the  eighth  ?  What 
is  the  ninth  ?    What  is  the  tenth? 

Fig.  127.  What  is  the  eleventh  ?    What  is  the  twelfth  ? 


Art.  265. — W^hat  is  the  universal  discharger  ?    What  is  its  use  ? 

Fig.  128.  W^hat  figure  represents  it?  Of  what  does  it  consist?  What 
is  necessary  in  using  the  universal  discharger?  W^hat  is  effect- 
ed by  this  means  ?  What  experiments  are  shown  by  means 
of  the  universal  discharger  ?  How  must  the  substance  be  placed  ? 
How  may  ether  or  alcohol  be  inflamed? 


Art.  266. — What  are  the  electrical  bells? 

Fig.  129    What  figure  represents  them?    What  are  they  designed  to 
show  ?    How  are  they  to  be  applied  ? 

Fig.  130.   Explain  the  spiral  tube. 


Fig.  125. 


Fig,  130. 


360 


QUESTIONS    FOR  EXAMIXATIOX. 


1 


Fig.  131.   Explain  the  hydrogen  pistol. 
Fig.  132,   What  does  Fig.  132  represent? 
Fig.  133.  Explain  Fig.  133. 

Explain  experiment  one. 
Fig.  134.  Explain  the  second  experiment.  The  third. 
Fig.  135.  Explain  the  fourth  experiment.  The  fifth.  The  sixth.  The 
seventh.  What  is  lightning  ?  What  is  thunder  ?  How  is 
the  aurora  borealis  supposed  to  be  caused?  Howls  the  elec- 
tricity which  a  body  manifests  by  being  brought  near  to  an 
excited  body,  without  receiving  a  spark  from  it,  said  to  be 
acquired?  When  an  insulated,  but  unelectrified  conductor, 
is  brought  near  an  insulated  charged  conductor,  what  is  said 
with  regard  to  the  end  near  the  excited  conductor  ?  W^iat 
example  is  given  to  illustrate  this  ?  Why  are  square  rods 
better  than  round  ones  to  conduct  electricity  silently  to  the 
gromid,  and  thus  protect  buildings  from  lightning?  How  far 
beyond  the  rod  do  lightning-rods  afford  protection?  In  what 
way  are  the  most  approved  hghtning-rods  constructed  ?  What 
is  remarked  with  regard  to  the  terms  negative  and  positive  ? 
How  can  this  be  illustrated?  What  is  said  with  regard  to 
the  time  the  electric  fluid  occupies  in  its  passage  through  its 
circuit?  What  example  is  given  to  show  that  the  fluid  prefers 
the  best  conductors?  In  what  different  ways  does  the  electric 
fluid  sometimes  pass  in  thunder-storms?  Why  is  it  unsafe, 
during  a  thunder-storm,  to  take  shelter  under  a  tree,  or  to  hold 
in  the  hand  any  edge-tools  ?  What  position  is  the  safest  in  a 
thunder-storm?  When  is  there  no  danger  to  be  apprehended 
from  the  lightning?  By  whom  were  lightning-rods  first  pro- 
posed? W^ho  first  discovered  that  thunder  and  lightning  are 
the  efiects  of  electricity? 


GALYAXISM,  OR  VOLTAIC  ELECTRICITY. 

Art.  26 T. — What  is  Galvanism?  By  whom  and  when  was  galvanism 
discovered  ?  What  led  to  the  discovery  ?  How  is  electricity 
generally  produced  ?  How  is  galvanic  electricity  produced  ? 
How  does  the  motion  of  the  galvanic  fluid,  excited  by  galvanic 
power,  differ  from  that  explained  in  the  science  of  Electricity? 
Wliat  bodies  are  most  easily  affected  by  the  galvanic  fluid? 

Art.  268. — How  is  the  galvanic  fluid  or  influence  excited?  What  il- 
lustrations of  this  are  given  ? 


UUKri'PlONS   FOR  EXAMINATrON. 


361 


Art.  — Into  what  are  the  conductors  of  the  galvanic  fluid  divided? 

What  substances  are  perfect  conductors?  What  substances 
are  imperfect  conductors  ? 

Art.  2^0. — What  is  necessary  in  order  to  produce  galvanic  action? 

Art.  27  !• — Of  what  is  the  simplest  galvanic  circle  composed?  What 

process  is  usually  adopted  for  obtaining  galvanic  electricity? 
Fig.  136.  Illustrate  this  by  Fig.  136.    What  effect  will  be  produced  if, 


Fig.  131.  Fig.  133. 


16 


362 


aUESTIOXS   FOR  EXAMINATION. 


instead  of  allowing  the  metallic  plates  to  come  into  direct  con- 
tact, the  commmiication  between  them  be  effected  by  wires? 
How  many  parts  are  there  in  the  above  arrangement?  What 
are  they?  What  effect  does  the  acid  produce?  What  is  the 
electrical  state  of  the  zinc  ?  Of  the  copper  ?  What  are  the 
arrows  in  Fig.  136  designed  to  show  ?  Where  must  the  sub- 
stance, to  be  submitted  to  the  action  of  the  fluid,  be  placed  ? 
What  is  said  of  the  electrical  effects  of  a  simple  galvanic  circle  ? 
What  examples  are  given  illustrating  the  operation  of  simple 
galvanic  circles? 


Art.  2T2. — How  may  the  galvanic  effects  of  the  simple  circle  be  in- 
creased?   What  are  compound  galvanic  circles? 


Art.  2T3.— Of  what  does  the  voltaic  pile  consist? 

Fig.  137.  AMiat  does  Fig.  137  represent?  How  may  a  voltaic  pile  be 
constructed  ?  Can  any  other  metal  be  used  ?  What  are  the 
arrows  in  the  figure  designed  to  show  ? 


Art.  2  T 4.— What  is  the  voltaic  batters- ?  ^^Tiat  is  said  with  regard  to 
the  electricity  excited  by  the  battery  ? 

Fig.  138.  VTho-t  does  Fig.  138  represent  ?  Of  what  does  the  voltaic  bat- 
tery consist  ?  How  is  the  communication  between  the  first  and 
last  plates  made  ?  Where  must  the  substance,  which  is  to  be 
submitted  to  galvanic  action,  be  placed  ?  How  can  a  compound 
battery  of  great  power  be  obtained  ? 

Fig.  139.  ^^^lat  does  Fig.  139  represent?  Of  what  does  this  battery  con- 
sist? How  can  the  electric  shock  from  the  voltaic  battery  be 
received  by  any  number  of  persons  ? 

Fig.  140,  What  figure  represents  Smee's  galvanic  batter)^?  Describe  it 
What  hquid  is  employed  for  this  batter)^  ? 

Fig.  141.  Describe  Fig.  141. 

Figs.  142,  143.    Describe  the  sulphate  of  copper  battery. 
  '  


Fig.  142. 


364 


aUESTIONS   FOR  EXAMINATION. 


Fig.  144.  Describe  Grove's  battery.  Which  is  the  most  powerful  battery  ? 
For  what  is  it  used  ? 

Art.  — What  effects  may  be  produced  by  a  spark  from  a  voltaic 

battery?  What  precaution  is  taken  in  regard  to  the  lines  of 
the  circuit  ? 

Art.  2T6. — In  how  many  ways  does  the  electricity  produced  by  the  gal- 
vanic or  voltaic  battery  differ  from  that  obtained  by  the  ordinary 
electrical  machine?  What  is  the  first?  What  is  here  meant 
by  intensity  ?  How  does  the  quantity  of  electricity  obtained  by 
galvanic  action  compare  with  that  obtained  by  the  machine  ? 
To  what  may  the  action  of  the  electrical  machine  be  compared  ? 
To  what  may  the  galvanic  action  be  compared  ?  What  is  the 
second  way  in  which  they  differ?  What  is  the  third?  What 
is  said  in  the  note  with  regard  to  the  third  circumstance  in 
which  the  electricity  obtained  by  the  ordinary  electrical  ma- 
chine differs  from  that  produced  by  the  galvanic  battery  ?  What 
is  said  of  the  effects  of  this  continued  current  on  the  bodies 
subjected  to  its  action? 

Art.  — On  what  does  the  effect  of  the  voltaic  pile  on  the  body  de- 

pend? What  facts  in  common  life  does  galvanism  explain? 
On  what  does  the  effects  of  galvanic  action  depend  ?  What  are 
batteries  constructed  of  large  plates  sometimes  called  ?  Why  ? 
Describe  them.  Upon  what  principle  is  the  caloriraotor  con- 
structed ?    Are  galvanic  batteries  of  value  ? 

MAGNETISM  AND  ELECTRO-MAaNETISM. 

Art.  2T8. — Of  what  does  Magnetism  treat?  How  many  kinds  of  mag- 
nets are  there  ?  What  are  they  ?  What  is  the  native  magnet  ? 
What  property  does  it  possess?  What  is  an  artificial  magnet? 
What  magnet  is  preferred,  for  all  purposes  of  accurate  experi- 
ment ?    How  can  an  artificial  magnet  be  made  ? 

Art.  3T9. — What  is  the  first  property  of  the  magnet?  Second?  Third? 
Fourth  ? 

Art.  2S0» — What  is  meant  by  the  polarity  of  a  magnet  ?  Where  is  the 
attractive  power  of  a  magnet  the  strongest?  When  will  a 
magnet  assume  a  position  directed  nearly  north  or  south  ?  What 
is  the  north  pole  of  the  magnet  ?  What  is  the  south  pole  ?  In 
what  ways  can  a  magnet  be  supported  so  as  to  enable  it  to 
manifest  its  polarity  ?, 

Art.  281. — How  do  the  same  and  different  poles  of  a  magnet  affect 
each  other?  What  is  said  with  regard  to  the  attraction  of 
magnets,  whether  native  or  artificial  ?    What  analogy  is  there 


aUESTlONS  FOR  EXAMINATION. 


365 


between  the  attractive  and  tepnlsive  powers  of  the  different 
kinds  of  electricity,  and  the  northern  and  southern  polarities  of 
the  magnet? 

Art.  282. — Can  a  magnet  communicate  its  properties  to  other  bodies? 

To  what  substances,  only,  can  these  properties  be  conveyed  ? 
Of  what  substances  are  all  natural  and  artificial  magnets,  as 
well  as  the  bodies  on  which  they  act,  composed?  How  can 
the  powers  of  a  magnet  be  increased?  What  is  a  horse-shoe 
magnet?  How  can  it  be  made  to  sustain  a  considerable 
weight  ?  What  is  this  bar  called  ?  How  does  soft  iron  differ 
from  hardened  iron,  with  respect  to  its  acquiring  and  losing  the 
magnetic  power? 

Art.  283. — What  effect  is  produced  when  a  magnet  is  broken  or  di- 
vided ?    Why  is  this  a  remarkable  circumstance  ? 

Art.  284:. — Where  does  the  magnetic  power  of  iron  or  steel  wholly 
reside  ?  In  what  particulars  do  magnetism  and  electricity  re- 
semble each  other?  What  is  the  first?  What  is  the  second? 
What  is  the  third  ?    What  is  the  fourth  ? 

Art.  285. — What  effect  has  heat  on  the  power  of  the  magnet?  By 
what  is  the  magnetic  attraction  diminished  ?  What  effect  has 
electricity  on  the  poles  of  a  magnet  ?  What  effect  has  elec- 
tricity sometimes  on  iron  and  st§el  ? 

Art.  286. — What  proportion  do  the  effects  produced  by  two  magnets, 
used  together,  bear  to  that  of  either,  used  alone?  What  is 
meant  by  the  inclination  or  dipping  of  the  magnet  ? 

Art.  28T. — Does  the  magnet,  when  suspended,  invariably  point  to  the 
north  and  south  points?  What  advantage  has  the  science  of 
magnetism  rendered  to  commerce  and  navigation?  Of  what 
does  the  mariner's  compass  consist?  To  whom  is  the  invention 
of  the  mariner's  compass  usually  ascribed?  How  may  the 
value  of  this  discovery  be  estimated  ? 


Fig.  144. 


366 


aUESTIOXS  FOR  EXAMIXATIOX. 


Art.  288. — Where  are  the  north  and  south  poles  of  a  ma^et  the  most 
powerful  ?  What  effect  has  a  magnet  on  a  piece  of  iron,  when 
it  is  brought  sufficiently  near  to  it?  How  are  artificial  magnets 
niade?  Does  the  magnet  which  is  employed  in  magnetizing  a 
steel  bar  lose  any  of  its  power  by  being  thus  employed  ?  What 
is  a  magnetic  magazine  ?  Hjw  is  a  magnetic  needle  made  ? 
What  is  said  with  regard  to  a  horse-shoe  magnet  ?  How 
should  a  horse-shoe  magnet  be  kept  1 

Art.  289. — Of  what  does  Electro-Magnetism  treat  ?  What  is  the  elec- 
.  trie  current  ?  What  does  the  science  of  electro-magnetism 
explain?  What  is  the  difference  between  the  currents  in  the 
single  and  the  compound  circles  ?  What  is  it  thought  causes 
magnetic  attraction?  What  discover}' was  made  in  the  year 
1819  ?  By  whom  ?  What  further  discovery  was  made  soon 
after,  and  by  whom?  What  does  this  philosopher  maintain? 
How  many  kinds  of  electricity  are  there?  How  do  the  phe- 
nomena exhibited  in  these  five  kinds  of  electricity  differ  ?  Can 
magnetism  be  developed  in  steel  not  previously  possessing  it? 
Where  must  the  steel  be  placed  ?  What  property  has  the 
uniting  wire  ? 

Art.  290. — What  are  the  principal  facts  in  connexion  with  the  science 
of  electro-magnetism?  What  is  the  first?  What  is  the  sec- 
ond ?  Vrnat  is  the  third  ?  What  is  the  fourth  ?  What  is  the 
fifth  ?  What  is  the  sixth  ?  What  is  the  seventh  ?  Where 
have  the  bodies  been  supposed  to  be  placed.,  in  all  the  effects  of" 
electricity  and  galvanism  that  have  hitherto  been  described? 
What  is  the  eighth  fact  in  connexion  with  the  science  of  electro- 
magnetism?  What  is  the  ninth?  What  is  the  tenth?  How 
can  the  direction  of  the  electric  current  be  ascertained  ? 

Art.  291. — If  a  magnet  be  freely  suspended,  and  a  current  of  electricity 
be  passed  near  it,  what  direction  will  it  assume?  What  illus- 
tration of  this  is  given  ?  What  second  illustration  is  giveo  ? 
In  what  direction  will  the  pole  of  the  needle  next  to  the  nega- 
tive end  of  the  wire  move,  if  the  connecting  wire  be  placed 
below  the  plane  in  which  the  needle  moves,  and  parallel  with 
it?  What  is  said  with  regard  to  the  attractions  and  repul- 
sions ? 

Art.  292« — How  may  the  two  sides  of  an  unmagnetized  steel  needle 
become  endued  with  the  north  and  south  polarity  ?  Under 
what  circumstances  will  it  become  permanently  magnetic? 


aUESTIONS   FOR  EXAMINATION. 


367 


Art.  293. — How  can  magnetism  bo  communicated  to  iron  and  steel? 


How  can  the  effect  be  more  conveniently  produced  ?  What 
illustration  of  this  is  given?  What  is  the  helix?  Why  should 
the  wire,  which  forms  the  helix,  be  coated  with  some  non-con- 
ducting substance  ?  What  is  said  of  a  helix,  if  it  be  placed 
so  that  it  may  move  freely  ? 


Art.  294. — How  can  the  magnetic  needle  be  made  to  deviate  from  its 
proper  direction  ?    What  is  a  needle  thus  prepared  called  ? 

Art.  295 • — What  is  the  electro-magnetic  rotation?    What  illustration 
is  given  ? 

Fig.  145.  What  does  Fig.  145  represent?    Explain  the  figure.    How  is 


the  freedom  of  motion,  which  is  required  on  the  wire,  obtained  ? 
How  can  the  metallic  contact  which  is  required  be  obtained? 
If  the  poles  of  a  battery  be  connected  with  the  horizontal  ex- 
ternal wires,  c  c,  throughout,  what  direction  will  the  current  of, 
electricity  take  ?  Round  what  pole  will  the  moveable  part  of 
the  wire  rotate  ?    Round  what  will  the  magnetic  pole  rotate  ? 


Fig.  146.  What  does  Fig.  146  represent?    Of  vrhat  does  it  consist? 


How  will  the  cylinders  in  each  revolve,  if,  instead  of  a  bar- 
magnet,  a  horse-shoe  magnet  be  employed,  v^ith  an  apparatus 
on  each  pole  similar  to  that  which  has  now  been  described  ? 


Fig.  145. 


Fig.  146. 


L 


368 


aUESTIONS   FOR  EXAMINATION. 


Art.  296. — How  may  the  magnetizing  power  of  the  connecting  wires 
be  increased?  Is  a  single  circuit  preferable  to  a  straight 
wire? 


Fig.  147.  Explain  Fig.  147. 


Art.  29  — What  is  a  bar  called  that  is  temporarily  magnetized  ?  How 
do  you  ascertain  the  poles  of  an  electro-magnet? 


Art.  298. — How  have  magnets  of  great  power  been  formed?  What 
weight  was  the  magnet  constructed  by  Professor  Henry  and 
Dr.  Ten  Eyck  capable  of  supporting? 

Fig.  148.  Explain  the  heliacal  ring  and  Fig.  148. 


Art.  299. — How  are  bars  of  the  U  form  most  readily  magnetized? 


Fig.  147. 


16* 


370 


aUESTIONS  FOR  EXAMINATION. 


Fig.  149.  Explain  Fig.  149. 

Art.  300. — Explain  the  electro-magnetic  telegraph. 

Art.  301. — Of  what  does  magneto-electricity  treat?    How  are  electric 

currents  excited  ? 
Fig.  150.  Explain  the  magneto-electric  machine,  Fig.  150. 

Art.  3 O 2.— What  is  thermo-electricity  ?  To  what  do  the  magnets  owe 
their  peculiar  properties  ?  What  follows  from  this  ?  How  many 
states  of  electricity  are  there?  What  is  said  of  that  derived 
from  the  common  electrical  machine?  What  is  said  of  that 
derived  from  the  galvanic  apparatus?  What  is  said  of  the 
thermo-electric  currents? 


Fig.  149. 


aUESTIONS  FOR  EXAMINATION. 


371 


ASTRONOMY. 

Art.  ;J03. — Of  what  doos  Astronomy  treat  ?    What  is  said  of  the  earth  ? 

How  is  the  earth  known  to  be  round?    Where  is  it  situated? 

Art.  3()  l. — Of  what  does  the  solar  system  consist?  Wliat  are  tlie  stars 
supposed  to  be?  Do  we  see  more  by  the  aid  of  glasses  than 
without? 

Art.  305. — How  may  the  planets  be  distinguished  from  the  stars  ?  How 
are  the  planets  distinguished  from  the  Jixed  stars?  What  is 
the  meaning  of  the  word  planet?  Why  are  they  called  plan- 
ets? What  are  the  fixed  stars?  What  are  the  sun,  moon, 
planets,  and  fixed  stars  supposed  to  be  ?  Why  do  they  appear 
so  small?  What  has  been  stated  with  regard  to  the  attraction 
of  portions  of  matter?  Upon  what  force  does  this  attraction 
depend?  What  follows  from  attraction  being  mutual?  What 
direction  do  bodies  take  when  actuated  by  several  forces?  Is 
this  true  with  regard  to  the  heavenly  bodies?  What  is  the 
centre  of  the  solar  system  ?  What  is  said  of  the  revolution  of 
the  planets  ? 

Art.  306. — What  are  the  paths  in  which  the  planets  move  around  the 
sun  called  ?  Around  what  do  the  planets  revolve  ?  What  is 
a  year  on  each  planet?  How  long  is  the  year  of  the  planet 
Mercury?  How  long  is  the  planet  Venus  performing  her  revo- 
lution around  the  sun  ?  How  long  is  the  earth  performing  her 
revolution  around  the  sun  ?  What  is  the  length  of  the  year  on 
the  planet  Mercury?  Venus?  Earth?  Mars?  Vesta?  Juno? 
Ceres?  Pallas?  Jupiter?  Saturn?  Herschel  ?  Neptune? 
Of  what  form  are  the  orbits  of  the  planets?  What  is  meant 
by  the  mean  distance?  What  planets  are  called  inferior? 
Why?  What  planets  are  called  superior ?  Why?  What  is 
the  distance  of  the  planet  Mercury  from  the  sun?  Venus? 
Earth?  Mars?  Vesta?  Juno?  Ceres?  Pallas?  Jupiter? 
Saturn?  Herschel?  Neptune?  Have  the  planets  any  mo- 
tion besides  that  around  the  sun  ?  What  is  the  time  in  which 
they  turn  upon  their  axes  called?  What  is  the  length  of  a 
day  on  the  planet  Mercury  ?  Venus?  Earth?  Mars?  Vesta? 
Juno?  Ceres?  Pallas?  Jupiter?  Saturn?  Herschel?  Nep- 
tune? 


Art.  sot. —What  is  the  diameter  of  the  Sun?    Mercury?  Venus? 

Earth?  Mars?  Vesta?  Juno?  Ceres?  Pallas?  Jupiter? 
Saturn?    Herschel?    The  Moon? 


372 


aUESTIOXS   FOR  EXAMIXATIOX. 


Fig.  151.  What  does  Fig.  151  represent  ?    What  illustration  of  the  com- 


parative size  and  distance  of  the  bodies  of  the  solar  system  is 
given  ?  What  is  necessary  in  order  to  imitate  the  motions  of 
the  planets  in  the  above-mentioned  orbits  ? 


Art.  308. — What  is  the  echptic?    Why  is  it  called  the  ecliptic? 
Art.  309« — What  is  the  zodiac?    Why  is  it  called  the  zodiac  ? 

Art.  310. — What  are  the  names  of  the  twelve  constellations?  How 

many  degrees  does  each  sign  contain? 

Art.  311. — Are  the  orbits  of  the  other  planets  in  the  same  plane  with 
that  of  the  earth  ? 

Fig.  152.   What  does  Fig.  152  represent?    What  are  the  nodes  of  a 
planet? 

Fig.  153.    What  does  Fig.  ]53  represent? 

Art.  3  1 2. — When  is  a  planet  said  to  be  in  any  particular  constellation  ? 
Art.  313. — What  do  the  perihelion  and  aphelion  of  a  heavenly  body 


express?  When  is  a  body  said  to  be  in  its  perihelion?  When 
is  a  body  said  to  be  in  its  aphelion  ?  How  much  nearer  is  the 
earth  to  the  sun  in  its  perihelion  than  in  its  aphelion  ?  When 
is  a  planet  said  to  be  in  its  inferior  conjunction  ?  When  is  it 
said  to  be  in  its  superior  conjunction?  When  is  it  said  to  be  in 
opposition  ? 


Art.  314. — What  do  the  apogee  and  perigee  of  a  heavenly  body  express? 

When  is  a  body  said  to  be  in  its  perigee?  When  is  it  said  to 
be  in  its  apogee  ? 


Fig.  151. 


Fig.  152. 


Fig.  153 


374  auESTioNs  for  examination. 


Art.  315, — In  what  sign  is  the  perihelion  of  the  planet  Mercury?  Ve- 
nus? Earth?  Mars?  Vesta?  Juno?  Ceres?  Pallas?  Ju- 
piter?   Saturn?    Georgium  Sidus? 

Art  316. — What  is  said  with  regard  to  the  axes  of  the  planets  in  their 
revolution  around  the  sun  ?  What  does  this  inclination  of  their 
axes  cause  ? 

Art.  3 1  T  • — What  is  said  with  regard  to  the  motion  of  the  heavenly 
bodies?  When  do  they  move  with  the  greatest  velocity? 
When  is  their  motion  the  slowest  ? 

Art.  3 1 8.— What  is  Kepler's  law  ? 

Fig.  154.  Illustrate  this  by  Fig.  154.  Explain,  by  Fig.  154,  the  reason 
why  the  earth,  or  any  other  heavenly  body^  moves  with  a 
greater  degree  of  velocity  in  its  perihelion  than  in  its  aphelion. 
What  is  said  of  the  motion  of  the  heavenly  bodies  from  peri- 
helion to  aphelion?  What  is  their  motion  from  aphelion  to 
perihelion?  When  is  their  velocity  the  greatest?  How  much 
longer  is  the  earth  in  performing  the  aphelion  part  of  its  orbit 
than  the  perihelion  part  ? 

Art.  319.— How  much  nearer  is  the  earth  to  tne  sun  in  winter  than  in 
summer  ? 

Art.  3  20. — What  follows  from  the  inclination  of  the  earth's  axis,  with 
regard  to  the  direction  of  the  sun's  rays  ?  When  is  the  heat  al- 
ways the  greatest?  What  is  said  of  oblique  rays?  What  is 
the  reason  that  the  heat  is  greater  in  summer  than  in  winter? 

Fig.  155.  Illustrate  this  by  Fig.  155.  How  is  the  earth  situated  with  re- 
gard to  its  distance  from  the  sun  in  winter?  What  illustration 
of  oblique  and  perpendicular  rays  is  given  in  the  note?  Why 
is  it  generally  cooler  early  in  the  morning  and  late  in  the  after- 
noon than  at  noon  ?  Why  is  the  heat  the  greatest  at  about 
three  o'clock  ?  What  causes  the  variety  of  climate  in  different 
Darts  of  the  earth?  W^here  does  the  sun  always  shine  in  a  ver- 
tical direction  ?  What  would  follow  were  the  axis  of  the  earth 
perpendicular  to  its  orbit?  What  causes  the  variety  of  the  sea- 
sons, the  different  lengths  of  days  and  nights,  «fec.  ?  What  is 
necessary  in  order  to  understand  the  illustration  of  the  causes 
of  the  seasons? 

Fig.  156.  Explain  Fig.  156.  What  are  the  poles?  Why  is  the  circle  IK 
called  the  tropic  of  Cancer?  What  is  the  meaning  of  the  word 
tropic  ?  Why  is  the  circle  L  M  called  the  tropic  of  Capricorn  ? 
What  are  the  tropics  ?  What  is  the  circle  E  F  called  ?  What 
does  it  represent?  What  is  the  circle  G  H  called  ?  What  does 
it  represent? 


Fig.  154. 
D  F 


Fig.  156. 


376 


QUESTIONS    FOR    EX  A  .AITX  ATIOX. 


Fig.  157.  ^^^lat  does  Fig.  157  represent?  Explain  the  figure.  Explain, 
by  the  figure,  the  situation  of  the  earth  on  the  21st  of  June. 
\Vhat  causes  day  and  night  1  To  what  part  of  the  earth  is 
it  day?  To  what  part  is  it  night?  To  what  is  the  length  of 
the  day  in  proportion  ?  When  are  the  days  the  longest  ?  Why  ? 
When  are  they  the  shortest?  Why  ?  Explain,  by  the  figure, 
the  situation  of  the  earth  on  the  2'2d  of  September.  On  the  23d 
of  December.  On  the  20th  of  March.  What  follows  from 
the  changes  on  the  earth,  caused  by  the  inclination  of  the 
earth's  axis?  In  what  proportion  are  these  changes ?  What 
is  said  of  the  axis  of  the  planet  Jupiter  ?  Is  it  supposed  that 
the  sun,  planets,  and  stars  are  inhabited  ?  What  is  shown  by 
Fig.  157?  Where  are  these  points?  What  are  they  called? 
Which  is  the  vernal  equinox  ?  Which  the  autumnal  ?  What 
other  two  points  are  there?  Why  are  they  called  solstices? 
Where  are  these  points?  Which  is  the  summer  solstice? 
Which  the  winter  ? 

Art.  3  21.— What  is  said  of  the  s-un  ?  What  is  its  diameter?  How 
much  does  its  cubic  magnitude  exceed  that  of  the  earth  ?  How 
long  is  it  in  performing  its  revolution  around  its  axis?  How 
has  this  been  ascertained?  What  did  Dr.  Herschel  suppose 
these  spots  to  be  ?  What  is  the  zodiacal  light  ?  At  what  time 
is  it  most  distinct  ?    Where  is  it  constantly  visible  ? 

Art.  3  22  ^Miat  planet  is  nearest  to  the  sun  ?   Why  is  it  seldom  seen  ? 

What  is  said  of  the  heat  of  this  planet  ?  How  much  greater  is 
the  sun's  heat  m  Mercury  than  on  the  earth  ?  In  what  form 
does  water  exist  in  Mercury  ?  How  can  Mercur}-  be  recog- 
nised when  seen  ?  At  what  time  does  it  appear  ?  How  does 
Mercury  appear  when  viewed  through  a  telescope  ? 

Art.  3  23.  What  planet  is  nearest  to  the  earth?    When  is  Venus  called 

the  morning  star  ?  When  is  it  called  the  evening  star  ?  How 
much  greater  are  the  light  and  heat  at  Venus  than  that  at  the 
earth  ?  What  name  was  given  by  the  ancient  poets  to  Venus, 
when  morning  star?  What,  when  evening  star  ?  What  is  the 
greatest  distance  at  which  the  planets,  Mercuiy  and  Venus,  ever 
appear  from  the  sun?  What  is  meant  by  ihe  transit  of  these 
planets?  What  is  said  of  the  difFeront  appearances  which 
Venus  presents  ?  Why  can  we  not  see  the  planets  and  stars  in 
the  daytime? 

Art  3  24.— What  planet  is  next  to  Venus?  What  is  the  form  of  the 
earth?  How  much  larger  is  its  equatorial  diameter  than  its 
polar  ?  How  many  moons  has  the  earth  ?  What  is  the  diame- 
ter of  the  moon  ?   What  is  its  distance  from  the  earth  ?    What  is 


aUESTIONS   FOR  EXAMINATION. 


377 


the  length  of  a  day  at  the  moon  ?  How  long  is  it  in  performing 
its  revolution  around  the  earth  ? 

Art.  32S, — What  phases  does  the  earth,  when  viewed  from  the  moon, 
exhibit  ?  How  much  larger  does  the  earth  appear  than  the 
moon  ? 

Art.  326* — What  planet  is  next  to  the  earth?  What  renders  it  con- 
spicuous? What  is  supposed  to  cause  this  appearance?  How 
much  more  light  and  heat  does  the  earth  enjoy  than  Mars? 
When  were  the  asteroids  discovered  ?  By  whom,  and  in  what 
year  was  Vesta  discovered  ?  What  is  the  color  of  its  light  ? 
By  whom  and  when  was  Juno  discovered?  What  is  the  color 
of  its  light  ?  When  was  Pallas  discovered  ?  By  whom  ?  What 
is  said  of  its  atmosphere?  When  and  by  whom  was  Ceres 
discovered?  What  is  its  color?  What  is  said  in  the  note  with 
regard  to  these  planets? 

Art.  321 . — Which  of  the  planets  is  the  largest?  How  much  more  light 
and  heat  does  the  earth  enjoy  than  Jupiter?  How  many 
moons  has  this  planet  ?  What  is  the  distance  of  these  moons 
from  the  planet?    In  what  time  do  they  perform  their  revolu- 


Fig.  157. 


378  auESTiONS  for  examination. 


tions  around  the  planet  ?  How  does  the  size  of  these  moons 
compare  with  that  of  ours  ?  Why  has  Jupiter  no  sensible  vari- 
ety of  seasons  ?  Of  what  use  are  the  eclipses  of  Jupiter's  moons  ? 
How  long  is  light  in  coming  from  the  sun  to  the  earth  ?  How 
has  this  been  ascertained?  How  does  Jupiter  appear  when 
viewed  through  a  telescope  ? 

Art.  328. — How  does  Saturn  compare  in  size  with  the  other  planets  ? 

How  is  Saturn  distinguished  from  the  other  planets?  What 
is  said  of  these  rings  ?  How  much  longer  are  these  rings  in 
performing  their  revolution  around  the  planet  than  the  planet  is 
in  performing  its  revolution  on  its  axis  ?  What  is  the  breadth  of 
these  rings?  What  is  said  of  the  surface  of  Saturn?  How 
many  moons  has  Saturn?  How  may  Saturn  be  known? 
What  is  said  of  the  moons  of  Saturn?  Why  are  they  not  often 
eclipsed  ? 

Art.  3S9« — How  does  Uranus  compare  in  size  with  the  other  planets? 

How  does  the  light  and  heat  at  Uranus  compare  with  that  of 
the  earth  ?  By  whom  was  this  planet  discovered  ?  What 
name  did  he  give  it?  How  many  moons  has  Uranus?  By 
whom  were  they  discovered  ?  How  are  their  orbits  situated, 
with  regard  to  that  of  the  planet  ?  What  is  said  of  their  mo- 
tion ?  What  appears  to  be  a  general  law  of  satellites  ?  What 
follows  from  this  with  regard  to  the  appearances  which  the  in- 
habitants of  the  secondary  planets  must  observe? 

Art.  330. — Who  discovered  the  planet  Neptune  ? 

Art.  331. — What  is  the  meaning  of  the  word  comet?  To  what  class  of 
bodies  is  this  name  given  ?  Of  what  do  these  bodies  appear  to 
consist?  What  is  the  number  of  comets  that  have  occasionally 
appeared?  What  discoveries  have  been  made  concerning  98  of 
them?  What  is  the  result?  What  is  the  form  of  the  orbits  of 
comets  ?  What  is  said  of  the  motion  of  comets  when  in  peri- 
helion ?  What  did  Newton  calculate  the  velocity  of  the  comet 
of  1680  to  be  in  an  hour?  For  what  was  this  comet  remarka- 
ble ?  What  is  said  of  the  luminous  stream  of  a  comet  as  it 
approaches  and  recedes  from  the  sun?  What  did  Newton, 
and  some  other  astronomers,  consider  the  tails  of  comets  to  be  ? 
What  is  said  in  the  note  with  regard  to  comets  ?  Who  were 
the  first  astronomers  that  successfully  predicted  the  return  of  a 
comet  ?  What  is  the  periodical  time  of  Halley's  comet  ?  Of 
Encke's?    Of  Biela's? 

Art.  33S. — Into  how  many  magnitudes  are  the  stars  classed?  Of  what 
magnitude  are  the  largest?  Of  what  are  the  smallest  ?  What 
are  telescopic  stars  ?    Why  cannot  the  distance  of  the  fixed 


QUEfcTIONS   FOR    K  X  A  MI  N  AT  K  >N.  371) 


stars  ho  dottM-iuiiuul  I  To  what  is  tho  (litlonMJCO  in  thoir  uppa- 
roiit  inagnitudoB  supposed  to  be  owing?  Have  Die  Htars  any 
motion  ? 


Art.  333. — Wlmt  is  the  Galaxy?    Of  w^hat  is  it  supposed  to  consist? 

How  did  tlio  ancients  divide  the  stars?  What  was  tho  number 
of  constellations  among  the  ancients?  How  many  have  been 
added  by  the  moderns  ?  How  are  the  stars  designated  on  the 
celestial  *rlobe  ?  What  is  the  situation  of  each  constellation 
now  ?  Illustrate  this.  What  is  the  cause  of  this  difference  ? 
Why  do  we  not  see  the  stars,  and  other  heavenly  bodies,  in 
their  true  situation?  How  can  a  star  be  seen  in  its  true  situa- 
tion? What  is  meant  by  the  aberration  of  light?  What  is 
necessary  to  be  taken  hito  consideration,  in  determining  the 
true  place  of  the  celestial  bodies  ?  What  effect  has  this  prop- 
erty of  the  atmosphere  on  the  length  of  the  days  ? 


Art.  334. — What  is  the  parallax  of  a  heavenly  body? 

Fig.  158.  Explain  Fig.  158.   What  appears  from  this  ?   What  allowance 
must  also  be  made  ? 


Art.  3  3  5. — Is  the  moon  a  primary  or  secondary  planet?  How  long 
is  it  in  performing  its  revolution  about  the  earth  ?  What  is  its 
distance  from  the  earth  ?  What  is  the  most  obvious  fact  in 
relation  to  the  moon?  How  is  this- caused?  What  kind  of  a 
body  is  the  moon  ?    By  what  light  does  it  shine  ? 


Fig.  158. 


380  QUESTIONS    FOR  EXAMINATION. 


Fig.  159.  Explain  Fig.  159.  How  does  the  moon  appear  when  viewed 
through  a  telescope?  What  causes  the  difference  in  the  rising 
of  the  moon  ?  What  is  the  mean  difference  in  the  rising  of  the 
moon?  What  is  the  harvest  moon?  What  is  the  hunter's 
moon?    When  are  the  moons  always  the  most  beneficial  ? 


Art.  336. — What  are  tides  ?    By  what  are  they  occasioned  ? 


Fig.  160.  Explain  the  theory  from  Fig.  160.  What  is  the  greatest  dis- 
tance of  the  moon  from  the  equator?  Where  are  the  tides 
highest?  Why?  Has  the  sun  any  effect  on  the  tides?  What 
are  spring  tides  ?   When  do  they  occur  ? 

Fig.  161.  What  are  neap  tides?    When  do  they  take  place  ? 


Art.  33T. — What  is  an  eclipse  ?  When  does  an  eclipse  of  the  sun  take 
place  ?  When  does  an  eclipse  of  the  moon  take  place  ?  What 
is  necessary  at  the  time  of  an  eclipse  ?  How  often  would  there 
be  an  eclipse,  if  the  moon  went  round  the  earth  in  the  same 
plane  in  which  the  earth  goes  round  the  sun  ?  Why  ?  What 
is  the  inclination  of  the  moon's  orbit  to  the  ecliptic  ?  What 
•  is  the  apparent  diameter  of  the  sun  and  moon  ?    What  follows 

from  this?  When  is  the  sun  eclipsed?  When  the  moon? 
Does  an  eclipse  happen  every  time  there  is  a  full  or  new 
moon  ?  What  must  the  shadows  of  these  bodies  always  be  ? 
Why? 

Fig.  162.  Explain  Fig.  162.    When  is  an  eclipse  called  annular? 


c 


382  auESTioxs  for  examination.  | 


Fig.  163.  Explain  by  Fior.  ^63.  What  is  a  penumbra  ?  Why  are  eclipses 
of  the  moon  visible  to  more  inhabitants  than  those  of  the  sun  ? 
Wliy  is  a  lunar  eclipse  visible  to  all  to  whom  the  moon  is  visi- 
ble at  the  time  ?  What  is  said  of  the  earth's  shadow  ?  Ex- 
plain by  the  figure.  Into  what  are  the  diameters  of  the  sun 
and  moon  supposed  to  be  divided  ?  How  many  digits  are  these 
bodies  said  to  have  eclipsed  ?  How  often  must  there  be  an 
eclipse  of  the  sun  ?  What  is  the  greatest  number,  of  both  lunar 
and  solar  eclipses,  that  can  take  place  in  a  j^ear?  What  is 
the  usual  number  ?  What  is  said  of  the  eclipse  of  the  sun  in 
1806  ? 

Art.  338.— What  is  time  called  when  calculated  by  the  sun?  What 
is  sidereal  time  ?  How  much  longer  is  the  sidereal  year  than 
the  solar?  How  is  a  solar  year  measured  ?  What  is  the  length 
of  a  solar  year  ?  Why  is  a  day  added  every  fourth  year,  to 
the  year  ?  How  is  a  sidereal  year  measured  ?  What  is  the 
precession  of  the  equinoxes  ?  What  change  has  this  circum- 
stance caused,  with  regard  to  the  situation  of  the  constella- 
tions ?  Can  true  equal  time  be  measured  by  the  sun  ?  Why  ? 
At  what  periods  of  the  3^ear  do  the  sun  and  a  perfect  clock 
agree  ?  What  is  the  greatest  difference  between  true  and  ap- 
parent time  ? 

Fig.  164.  What  does  Fig.  164  represent? 


DAVIES'  SYSTEM  OF  MATHEMATICS. 


MATOEMAf  I(DAL  WmK 

DESIGNED  FOR  SCHOOLS,  ACADEMIES,  AND  COLLEGES. 

BY  CHARLES  DAVIES,  IL.D. 


PUBLISHED  BY  A.  S.  BARNES  &  CO.. 

51  JOHN-STREET,  NEW  YORK. 


ELEMENTARY  COURSE. 

Price 

l^AVIES'  PRIMARY  TABLE  BOOK   12 

DAVIES'  FIRST  LESSONS  IN  ARITHMETIC   19 

DAVIES'  SCHOOL  ARITHMETIC  New  Edition,  38 

KEY  TO  DAVIES'  SCHOOL  ARITHMETIC   38 

DAVIES'  UNIVERSITY  ARITHMETIC  Umo.  sheep,  75 

KEY  TO  DAVIES'  UNIVERSITY  ARITHMETIC   50 

DAVIES'  ELEMENTARY  ALGEBRA  l2mo.  sheep,  84 

KEY  TO  DAVIES'  ELEMENTARY  ALGEBRA   50 

DAVIES'  ELEMENTARY  GEOMETRY  12mo.  sheep,  75 

DAVIES'  DRAWING  AND  MENSURATION  l2mo.  sheep,  84 

ADVANCED  COURSE. 

DAVIES'  BOURDON'S  ALGEBRA— Being  an  abridgment  of  the  work  ^"*'** 
of  M.  Bourdon  8vo.  sheep,    1  50 

DAVIES'  LEGENDRE'S  GEOMETRY  AND  TRIGONOMETRY— 

Being  an  abridgment  of  the  work  of  M.  Legendre  8vo.  sheep,    1  50 

DAVIES'  ELEMENTS  OF  SURVEYING-With  a  description  and  Plates 

of  the  Theodolite,  Compass,  Plane  Table,  and  Level  8vo.  sheep,    1  50 

DAVIES'  ANALYTICAL  GEOMETRY— Embracing  the  Equations  of 
the  Point  and  Straight  Line ;  a  System  of  Conic  Sections  ;  the  Equa- 
tions of  the  Lme  and  Plane  in  Space,  &c  8vo.  sheep,    1  50 

DAVIES'  DIFFERENTIAL  AND  INTEGRAL  CALCULUS— Embra- 
cing the  Rectification  and  Quadrature  of  Curves,  the  Mensuration  of . 
Surfaces,  and  the  Cubature  of  Solids  8vo.  sheep,    1  50 

DAVIES'  DESCRIPTIVE  GEOMETRY— With  its  application  to  Spheri- 
cal Projections  8vo.  sheep,  2  00 

DAVIES'  SHADES,  SHADOWS,  AND  LINEAR  PERSPECTIVE— 

With  numerous  Plates  8vo.  calf  back,    2  50 


Davies^  System  of  Mathematics, 


TO  THE  FRIENDS  OF  EDUCATION. 

The  publishers  of  this  series  of  mathematical  works  by  Professor 
Charles  Davies,  beg  leave  respectfully  to  ask  of  teachers  and  the 
friendo  of  education  a  careful  examination  of  these  works.  It  is 
not  their  intention  to  commend,  particularly,  this  Course  of  Math- 
ematics to  public  favor;  and  especially,  it  is  not  their  design  to 
disparage  other  works  on  the  same  subjects.  They  wish  simply 
to  explain  the  leading  features  of  this  system  of  Text-Books — the 
place  which  each  is  intended  to  fill  in  a  system  of  education — the 
general  connection  of  the  books  with  each  other — and  some  of  the 
advantages  which  result  from  the  study  of  a  uniform  series  of  math- 
ematical works. 

It  may,  perhaps,  not  be  out  of  place,  first,  to  remark,  that  the 
author  of  this  series,  after  graduating  at  the  Military  Academy, 
entered  upon  the  duties  of  a  permanent  instructor  in  that  institution 
in  the  year  1816,  and  was  employed  for  the  twenty  following  years 
in  the  departments  of  scientific  instruction.  At  the  expiration  of 
that  period  he  visited  Europe,  and  had  a  full  opportunity  of  com- 
paring the  systems  of  scientific  instruction,  both  in  France  and 
England,  with  that  which  had  been  previously  adopted  at  the  Mili- 
tary Academy. 

This  series,  combining  all  that  is  most  valuable  in  the  various 
methods  of  European  instruction,  improved  and  matured  by  the 
suggestions  of  more  than  thirty  years'  experience,  now  forms  the 
only  complete  consecutive  course  of  Mathematics.  Its  methods, 
harmonizing  as  the  works  of  one  mind,  carry  the  student  onward 
by  the  same  analogies  and  the  same  laws  of  association,  and  are  cal- 
culated to  impart  a  comprehensive  knowledge  of  the  science,  com- 
bining clearness  in  the  several  branches,  and  unity  and  proportion 
in  the  whole.  Being  the  system  so  long  in  use  at  West  Point,  and 
through  which  so  many  men,  eminent  for  their  scientific  attain- 
ments, have  passed,  it  may  be  justly  regarded  as  our  National 
System  of  Mathematics.  Scholars  and  students  who  have  pur- 
sued this  course,  will  everywhere  stand  on  the  highest  level  with 
reference  to  the  estimates  which  themselves  and  others  will  form 
of  this  part  of  their  education. 

The  series  is  divided  into  three  parts,  viz.  :  First — Arithmeti- 
cal Course  for  Schools.  Second — Academical  Course.  Third 
— Collegiate  Course 

^3) 


Tlio  Arillinioliral  Toiirse  for  Schools. 

I.  PRIMARY  TABLE-BOOK. 
II.  FIRST  LESSONS  IN  ARITHMETIC. 
III.  SCHOOL  ARITHMETIC.    (Key  separate  J 


PRIMARY  TABLE-BOOK. 

The  leading  feaUire  of  the  plan  of  this  work  is  to  teach  the  reading  of  figure? 
that  is,  so  to  train  the  mind  that  it  sliall,  by  the  aid  of  the  eye  alone,  catch  in- 
stantly the  idea  which  any  combination  of  figures  is  intended  to  express. 

The  method  heretofore  pursued  has  airned  only  at  presenting  the  combinations 
by  means  of  our  common  language  :  this  method  proposes  to  present  them  pure- 
ly through  the  arithmetical  symbols,  so  that  the  pupil  shall  not  be  obliged  to  pause 
at  every  step  and  translate  his  conceptions  into  common  language,  and  then  re- 
translate them  into  the  language  of  arithmetic. 

For  example,  when  he  sees  two  numbers,  as  4  and  8,  to  be  added,  he  shall  not 
pause  and  say,  4  and  8  are  12,  but  shall  be  so  trained  as  to  repeat  12  at  once,  as  is 
always  done  by  an  experienced  accountant.  So,  if  the  difference  of  these  num- 
bers is  to  be  found,  he  shall  at  once  say  4,  and  not  4  from  8  leaves  4.  If  he  de- 
sires their  product,  he  will  say  32  ;  if  their  quotient,  2  :  and  the  same  in  all  simi- 
lar cases. 

FIRST  LESSONS  IN  ARITHMETIC. 

The  First  Lessons  in  .Arithmetic  begin  with  counting,  and  advance  step  by  step 
through  all  the  simple  combinations  of  numbers.  In  order  that  the  pupil  may  be 
impressed  with  the  fact  that  numbers  express  a  collection  of  units,  or  things  of 
the  same  kind,  the  unit,  in  the  beginning,  is  represented  by  a  star,  and  the  child 
should  be  made  to  count  the  stars  in  all  cases  where  they  are  used.  Having  once 
fixed  in  the  mind  a  correct  impression  of  numbers,  it  was  deemed  no  longer 
necessary  to  represent  the  unit  by  a  symbol ;  and  hence  the  use  of  the  star  was 
discontinued.  In  adding  1  to  each  number  from  1  to  10,  we  have  the  first  ten 
combinations  in  arithmetic.  Then  by  adding  2  in  the  same  way,  we  have  the 
second  ten  combinations,  and  so  on.  Each  ten  combinations  is  arranged  in  a 
separate  lesson,  throughout  the  four  ground  rules,  and  each  is  illustrated  either 
by  unit  marks  or  a  simple  example.  Thus  the  four  hundred  elementary  combi- 
nations are  presented,  in  succession,  in  forty  lessons, — a  plan  not  adopted  in  any 
other  elementary  book. 

SCHOOL  ARITHMETIC. 

This  work  begins  with  the  simplest  combination  of  numbers,  and''contains  all 
that  is  supposed  to  be  necessary  for  the  average  grade  of  classes  in  schools.  It 
is  strictly  scientific  and  entirely  practical  in  its  plan.  Each  idea  is  first  presented 
to  the  mind  either  by  an  example  or  an  illustration,  and  then  the  principle,  or 
abstract  idea,  is  stated  in  general  terms.  Great  care  has  been  taken  to  attain 
simplicity  and  accuracy  in  the  definitions  and  rules,  and  at  the  same  time  so  to 
frame  them  as  to  make  them  introductory  to  the  higher  branches  of  mathematical 
science.  No  definition  or  rule  is  given  until  the  mind  of  the  pupil  has  been 
brought  to  It  by  a  series  of  simple  inductions,  so  that  mental  training  may  begin 
with  the  first  intellectual  efforts  m  numbers 

4 


Davies^  System  of  Mathematics, 


The  Academic  Course. 

I.  THE  UNIVERSITY  ARITHMETIC.  (Key separate.) 
II.  PRACTICAL  GEOMETRY  AND  MENSURATION. 

III.  ELEMENTARY   ALGE3RA.  (Key  separate,) 

IV.  ELEMENTARY  GEOMETRY. 

V.  DAVIES'  ELEMENTS  OF  SURVEYING. 


Those  who  are  conversant  with  the  preparation  of  elementary 
text-books,  have  experienced  the  difficulty  of  adapting  them  to  the 
wants  which  they  are  intended  to  supply.  The  institutions  of  in- 
struction are  of  all  grades  from  the  college  to  the  district  school, 
and  although  there  is  a  wide  difference  between  the  extremes,  the 
level  in  passing  from  one  grade  to  the  other  is  scarcely  broken. 
Each  of  these  classes  of  semmaries  requires  text-books  adapted  to 
its  own  peculiar  wants  ;  and  if  each  held  its  proper  place  in  its 
own  class,  the  task  of  supplying  suitable  text-books  would  not  be 
so  difficult.  An  indifferent  college  is  generally  inferior,  in  the 
system  and  scope  of  instruction,  to  a  good  academy  or  high-school ; 
while  the  district-school  is  often  found  to  be  superior  to  its  neigh- 
boring academy. 

Although,  therefore,  the  University  Arithmetic  and  the  Practical 
Geometry  and  Mensuration,  have  been  classed  among  the  books 
appropriate  for  academies,  they  may  no  doubt  be  often  advantage- 
ously studied  in  the  common-school ;  so  also  wdth  the  iVlgebra  and 
Elementary  Geometry.  The  Practical  Geometry  and  Mensura- 
tion, containing  so  much  practical  matter,  can  hardly  fail  to  be  a 
useful  and  profitable  study. 

DAVIES'  UNIVERSITY  ARITHMETIC. 

The  scholar  in  commencing  this  work,  is  supposed  to  be  familiar  with  the  oper- 
ations in  the  four  ground  rules,  which  are  fully  taught  both  in  the  First  Lessons 
and  in  the  School  Arithmetic.  This  being  premised,  the  language  of  figures, 
which  are  the  representatives  of  numbers,  is  carefully  taught,  and  the  different 
significations  of  which  the  figures  are  susceptible,  depending  on  the  places  in 
which  they  are  written,  are  fully  explained.  It  is  shown,  for  example,  that  the 
simple  numbers  in  which  the  unit  increases  from  right  to  left  according  to  the 
scale  of  tens,  and  the  Denominate  or  Compound  Numbers,  in  which  it  increases 
according  to  a  different  scale ;  belong  in  fact  to  the  same  class  of  numbers,  and 
that  both  may  be  treated  under  a  common  set  of  rules.  Hence,  the  rules  for  No- 
tation, Addition,  Subtraction,  Multiplication,  and  Division,  have  been  so  con- 
structed as  to  apply  equally  to  all  numbers.  This  arrangement  is  a  new  one,  and 
is  deemed  an  essential  improvement  in  the  science  of  numbers 

la  developing  the  properties  of  numbers,  from  their  elementarj^  to  their  highest 
combinations,  great  labor  has  been  bestowed  on  classification  and  arrangement. 
It  has  been  a  leading  object  to  presciil  tht  entire  subject  of  arithmetic  as  forming 

',5 ' 


Davics''  System  of  Mallicmafics. 


a  series  of  depeiulent  and  connected  propositions;  so  that  the  pupil,  while  ac- 
quiring useful  and  practical  knowledge,  may  at  the  same  time  be  introduced  to 
those  b(!auliful  niethods  of  exact  reasoning  wlii(^li  science  alone  can  teach. 

Great  care  has  been  taken  to  demonstrate  fully  all  the  rules,  and  to  explain  the 
reason  of  every  process,  from  the  most  simple  to  the  most  difficult.  The  demon- 
stration of  the  rule  for  the  division  of  fractions,  on  page  147,  is  new  and  consid- 
ered valuable. 

The  properties  of  the  9's,  explained  at  page  93,  and  the  demonstration  of  the 
four  ground  rules  by  means  of  those  properties,  are  new  in  their  present  form, 
and  are  thought  worthy  of  special  attention. 

In  the  preparation  of  the  work,  another  object  has  been  kept  constantly  in 
view  ;  viz.,  to  adapt  it  to  the  business  wants  of  the  country.  For  this  purpose, 
much  pains  have  been  bestowed  in  the  preparation  of  the  articles  on  Weights 
and  Measures,  foreign  and  domestic— on  Banking,  Bank  Discount,  Interest,  Coins 
and  Currency,  Exchanges,  Book-keeping,  &c.  In  short,  it  is  a  full  treatise  on 
the  subject  of  Arithmetic,  combining  the  two  characteristics  of  a  scientific  and 
practical  work. 


Recommendation  from  the  Professors  of  the  Mathematical  Department  of  the 
United  States  Military  Academy 
In  the  distinctness  with  which  the  various  definitions  are  given— the  clear  and 
strictly  mathematical  demonstration  of  the  rules— the  convenient  form  and  well- 
chosen  matter  of  the  tables,  as  well  as  in  the  complete  and  much-desired  appli- 
cation of  all  to  the  business  of  the  country,  the  "  University  Arithmetic"  of 
Prof.  Davies  is  superior  to  any  other  work  of  the  kind  with  which  we  are  ac- 
quainted. These,  with  the  many  other  improvements  introduced  by  the  ad- 
mirable scientific  arrangement  and  treatment  of  the  whole  subject,  and  in  par- 
ticular those  of  the  generalization  of  the  four  ground  rules,  so  as  to  include 
"  simple  and  denominate"  numbers  under  the  same  head,  and  the  very  plain 
demonstration  of  the  rule  for  the  division  of  fractions— both  of  which  are,  to  us, 
original— m-ake  the  work  an  invaluable  one  to  teachers  and  students  who  are  de- 
sirous to  teach  or  study  arithmetic  as  a  science  as  well  as  an  art. 

(Signed,)  D.  H.  MAHAN,  Prof.  Engineering. 

W.  H.  C.  BARTLETT,  Prof.  Nat.  Phil. 
A.  E.  CHURCH,  Prof.  Mathematics. 
United  States  Military  Academy,  Jan.  18,  1847. 


PRACTICAL  GEOMETRY  AND  MENSURATION. 

The  design  of  this  work  is  to  alford  schools  ana  academies  an  Elementary 
Text  Book  of  a  practical  character.  The  introduction  into  our  schools,  within 
the  last  few  years,  of  the  subjects  of  Natural  Philosophy,  Astronomy,  Mineralo- 
gy, Chemistry,  and  Drawing,  has  given  rise  to  a  higher  grade  of  elementary 
studies ;  and  the  extended  application  of  the  mechanic  arts  calls  for  additional 
information  among  practical  men.  In  this  work  all  the  truths  of  Geometry  are 
made  accessible  to  the  general  reader,  by  omitting  the  demonstrations  altogether, 
and  relying  for  the  impression  of  each  particular  truth  on  a  pointed  question  and 
an  illustration  by  a  diagram.  In  this  way  it  is  believed  that  all  the  important 
properties  of  the  geometrical  figures  may  be  learned  in  a  few  weeks  ;  and  after 
these  properties  have  been  once  applied,  the  mind  receives  a  conviction  of  their 
truth  little  short  of  what  is  afforded  by  rigorous  demonstration.  The  work  is 
divided  into  seven  books,  and  each  book  is  subdivided  into  sections. 

In  Book  I.,  the  properties  of  the  geometrical  figures  ^re  explained  bv  questions 
&v.'A  illustrati.ons. 

(6> 


Davies'  System  of  Mathematics, 


In  Book  IT.  are  explained  the  construction  and  uses  of  the  various  scales ;  and 
also  the  construction  of  geometrical  figures.  It  is,  as  its  title  imports,  Practica. 
Geometry. 

Book  III.  treats  of  Drawing.  Section  I.,  of  the  Elements  of  the  Art ;  Section 
II.,  of  Topographical  Drawing  ;  and  Section  III.,  of  Plan  Drawing. 

Book  IV.  treats  of  Architecture—explaining  the  different  orders,  both  by  de- 
scriptions and  drawings. 

Book  V.  contains  the  application  of  the  principles  of  Geometry  to  the  Mensu- 
ration of  Surfaces  and  Solids.  A  separate  rule  is  given  for  each  case,  and  the 
whole  is  illustrated  by  numerous  and  appropriate  examples. 

Book  VI.  contains  the  application  of  the  preceding  Books  to  Artificers'  and  Me- 
chanics' work.  It  contains  full  explanations  of  all  the  scales— the  uses  to  which 
they  are  applied— and  specific  rules  for  the  calculations  and  computations  which 
are  necessary  in  practical  operations. 

Book  VII.  is  an  introduction  to  Mechanics.  It  explains  the  nature  and  proper- 
ties of  matter,  the  laws  of  motion  and  equilibrium,  and  the  principles  of  all  the 
simple  machines. 

ELEMENTARY  ALGEBRA. 
This  work  is  intended  to  form  a  connecting  link  between  Arithmetic  and  Alge 
bra,  and  to  unite  and  blend,  as  far  as  possible,  the  reasoning  on  numbers  with  the 
more  abstract  method  of  analysis.  It  is  intended  to  bring  the  subject  of  Algebru 
within  the  range  of  our  common- schools,  by  giving  to  it  a  practical  and  tangible 
form.  It  begins  with  an  introduction,  in  which  the  subject  is  first  treated  men- 
tally, in  order  to  accustom  the  mind  of  the  pupil  to  the  first  processes  ;  after 
which,  the  system  of  instruction  assumes  a  practical  form.  The  definitions  and 
rules  are  as  concise  and  simple  as  they  can  be  made,  and  the  reasonings  are  as 
clear  and  concise  as  the  nature  of  the  subject  will  admit.  The  strictest  scientific 
methods  are  always  adopted,  for  the  double  reason,  that  what  is  learned  should 
be  learned  in  the  right  way,  and  because  the  scientific  methods  are  generally  the 
most  simple. 

ELEMENTARY  GEOMETRY. 
This  work  is  designed  for  those  whose  education  extends  beyond  the  acquisi- 
tion of  facts  and  practical  knowledge,  but  who  have  not  the  time  to  go  through 
a  full  course  of  mathematical  studies.  It  is  intended  to  present  the  striking  and 
important  truths  of  Geometry  in  a  form  more  simple  and  concise  than  is  adoptea 
in  Legendre,  and  yet  prescribe  the  exactness  of  rigorous  reasoning.  In  this  sys- 
tem, nothing  has  been  omitted  in  the  chain  of  exact  reasoning,  nothing  has  been 
taken  for  granted,  and  nothing  passed  over  without  being  fully  demonstrated 
The  work  also  contains  the  applications  of  Geometry  to  the  Mensuration  of  Sur 
faces  and  Solids. 

SURVEYING. 

In  this  work  it  was  the  intention  of  the  author  to  begin  with  the  very  elemem& 
of  the  subject,  and  to  combine  those  elements  in  the  simplest  manner,  so  as  to 
render  the  higher  branches  of  Plane  Surveying  comparatively  easy.  All  the  in- 
struments needed  for  plotting  have  been  carefully  described,  and  the  uses  of  those 
required  for  the  measurement  of  angles  are  fully  explained.  The  Conventional 
Signs  adopted  by  the  Topographical  Bureau,  and  which  are  now  used  by  the  United 
States  Engineers  in  all  their  charts  and  maps,  are  given  in  full.  An  account  is  also 
given  of  the  manner  of  surveying  the  public  lands  ;  and  although  the  method  is  sim- 
ple, it  has  nevertheless  been  productive  of  great  results.  The  work  also  contains 
a  Table  of  Logarithms— a  Table  of  Logarithmic  Sines— a  Traverse  Table,  and  % 
Table  of  Natural  Sines— being  all  the  Tables  necessary  for  Practical  Surveying 

(7) 


Davics^  System  of  Mathematics. 


The  Collegiate  Course. 

I.  DAVIES"  BOURDON'S  ALGEBRA. 
II.  DAVIES'  LEGENDRE'S  GEOMETRY  AND  TRIGONOMETRY. 

III.  DAVIES'  ANALYTICAL  GEOMETRY. 

IV.  DAVIES'  DESCRIPTIVE  GEOMETRY. 

V.  DAVIES'  SHADES,  SHADOWS,  AND  PERSPECTIVE. 

VI.  DAVIES'  DIFFERENTIAL  AND  INTEGRAL  CALCULUS. 


The  works  embraced  under  the  head  of  the  "  Collegiate  Course," 
were  originally  prepared  as  text-books  for  the  use  of  the  Military 
Academy  at  West  Point,  where,  with  a  single  exception,  they  are 
still  used.  Since  their  introduction  into  many  of  the  colleges  of 
the  country,  they  have  been  somewhat  modified,  so  as  to  meet  the 
wants  of  collegiate  instruction.  The  general  plan  on  which  these 
works  are  written,  was  new  at  the  time  of  their  appearance.  Its 
main  feature  was  to  unite  the  logic  of  the  French  School  of 
Mathematics  with  the  practical  methods  of  the  English,  and  the 
two  methods  are  now  harmoniously  blended  in  most  of  our  systems 
of  scientific  instruction. 

The  introduction  of  these  works  into  the  colleges  was  for  a 
long  time  much  retarded,  in  consequence  of  the  great  deficiency  in 
the  courses  of  instruction  in  the  primary  schools  and  academies  : 
and  this  circumstance  induced  Professor  Davies  to  prepare  his 
Elementary  Course. 

The  series  of  works  here  presented,  form  a  full  and  complete 
course  of  mathematical  instruction,  beginning  with  the  first  com- 
binations of  arithmetic,  and  terminating  in  the  higher  applications 
of  the  Diflferential  Calculus.  Each  part  is  adapted  to  all  the 
others.  The  Definitions  and  Rules  in  the  Arithmetic,  have 
reference  to  those  in  the  Elementary  Algebra,  and  these  to  similar 
ones  in  the  higher  books.  A  pupil,  therefore,  who  begins  this 
course  in  the  primary  school,  passes  into  the  academy,  and  then 
into  the  college,  under  the  very  same  system  of  scientific  in- 
struction. 

The  methods  of  teaching  are  all  the  same,  varied  only  by  the 
nature  and  difficulty  of  the  subject.  He  advances  steadily  frora 
one  grade  of  knowledge  to  another,  seeing  as  he  advances  the  con 
nection  and  mutual  relation  of  all  the  parts  :  and  when  he  reacneji 
the  end  of  his  course,  he  finds  indeed,  that  "  science  is  but  know 
ledge  reduced  to  order 


Davies'  System  of  Mathematics. 


DAVIES'  BOURDON. 

The  Treatise  on  Algebra  by  M.  Bourdon,  is  a  work  of  singular  excellence 
and  merit.  In  France  it  is  one  of  the  leading  text-books.  Shortly  after  its  first 
publication  it  passed  through  several  editions,  and  has  formed  the  basis  of  every 
subsequent  work  on  the  subject  of  Algebra. 

The  original  work  is,  however,  a  full  and  complete  treatise  on  the  subject  of 
Algebra,  the  later  editions  containing  about  eight  hundred  pages  octavo.  The 
time  given  to  the  study  of  Algebra  in  this  country,  even  in  those  seminaries  where 
the  course  of  mathematics  is  the  fullest,  is  too  short  to  accomplish  so  volumin- 
ous a  work,  and  hence  it  has  been  found  necessary  either  to  modify  it,  or  to 
abandon  it  altogether.  The  Algebra  of  M.  Bourdon,  however,  has  been  regarded 
only  as  a  standard  or  model,  and  it  would  perhaps  not  be  just  to  regard  him  as 
responsible  for  the  work  in  its  present  form. 

In  this  work  are  united  the  scientific  discussions  of  the  French  with  the  prac- 
tical methods  of  the  English  school,  so  that  theory  and  practice,  science  and  art, 
may  mutually  aid  and  illustrate  each  other.  A  great  variety  of  examples  have 
also  been  added  in  the  late  editions. 

DAVIES*  LEGENDRE. 

Legendre's  Geometry  has  taken  the  place  of  Euclid,  to  a  great  extent,  both  in 
Europe  and  in  this  country.  In  the  original  work  the  propositions  are  not 
enunciated  in  general  terms,  but  with  reference  to,  and  by  the  aid  of,  the  par- 
ticular diagrams  used  for  the  demonstrations.  It  was  supposed  that  this  de- 
parture from  the  method  of  Euclid  had  been  generally  regretted,  and  among  the 
many  alterations  made  in  the  original  work,  to  adapt  it  to  the  systems  of  in- 
struction in  this  country,  that  of  enunciating  the  propositions  in  general  terms 
should  be  particularly  named  ;  and  this  change  has  met  with  universal  acceptance. 

To  the  Geometry  is  appended  a  system  of  Mensuration  of  Planes  and  Solids— 
a  full  treatise  on  Plane  and  Spherical  Trigonometry— and  a  table  of  Logarithms, 
and  Logarithmic  Sines,  Tangents,  and  Secants.  The  whole  forms  a  complete 
system  of  Geometry  with  its  applications  to  Trigonometry  and  Mensuration, 
together  with  the  necessary  tables. 

ANALYTICAL  GEOMETRY. 
This  work  embraces  the  investigation  of  the  properties  of  geometrical  figures 
oy  means  of  analysis.  It  commences  with  the  elementary  principles  of  the  sci- 
ence, discusses  the  Equation  of  the  Straight  Line  and  Circle— the  Properties  of 
the  Conic  Sections— the  Equation  of  the  Plane— the  Positions  of  Lines  in  Space, 
and  the  Properties  of  Surfaces. 

DESCRIPTIVE  GEOMETRY. 

Descriptive  Geometry  is  intimately  coimected  with  Architecture  and  Civil 
Engineering,  and  alfords  great  facilities  in  all  the  operations  of  Construction. 

As  a  mental  discipline,  the  study  of  it  holds  the  first  place  among  the  various 
branches  of  Mathematics. 

SHADES,  SHADOWS,  AND  PERSPECTIVE. 
This  work  embraces  the  various  applications  of  Descriptive  Geometry  to 
Drawing  and  Linear  Perspective. 

DIFFERENTIAL  AND  INTEGRAL  CALCULUS. 
This  treatise  on  the  DiiTerential  and  Integral  Calculus,  was  intended  to  supply 
the  higher  seminaries  of  learning  vvith  a  text-book  on  that  branch  of  science.  It 
is  a  work  after  the  French  methods  of  teaching,  and  in  which  the  aotation  of  thi 
French  school  is  adopted. 

(9) 


Davies''  Mathematical  Works, 


A  CATALOGUE 

OF  THE 

COLLEGES  AND  UOTYERSITIES 

THAT  HAVE  ADOPTED 

DAVIES'  MATHEMATICAL  WORKS, 

(THE  WHOLE  SERIES,  OR  IN  PART.) 


THE  UNITEi)  STATES  MILITARY  ACADEMY. 


Dartmouth  College, 

Hanover^ 

New  Hampshire 

University  of  Vermont, 

Burlington^ 

Vermont. 

Norwich  University, 

Norwich, 

Brown  University, 

Providence, 

Rhode  Island. 

Washington  College, 

Hartford, 

Connecticut. 

Wesley  an  University, 

Middletown, 

Columbia  College, 

New  York, 

New  York. 

Union  College, 

Schenectady, 

((  (( 

Hamilton  Lit.  and  Theol.  Instit, 

Hamilton, 

M  <« 

Geneva  College, 

Geneva, 

((  it 

University  of  New  York, 

New  York, 

College  of  New  Jersey, 

Princeton, 

New  Jersey. 

University  of  Pennsylvania, 

Philadelphia, 

Pennsylvania. 

Lafayette  College, 

Easton, 

tt 

Marshall  College, 

Mercersburg, 

it 

Dickinson  College, 

Carlisle, 

Jefferson  College, 

Canonshurg, 

Washington  College, 

Washington, 

Alleghany  College, 

Meadville, 

t< 

Western  University, 

Pittsburg, 

(( 

Newark  College, 

Newark, 

Delaware. 

Georgetown  College, 

Georgetown, 

DisT.  OF  Columbia- 

Columbian  College, 

Washington, 

St.  John's  College, 

Annapolis, 

Maryland, 

St.  Mary's  College, 

Baltimore, 

Mount  St.  Mary's  College, 

Emmettsburg, 

Washington  College, 

Lexington, 

Virginia. 

10 


Daoies^  Mathematical  Works. 


HsmpclBii-SydnBy  CollcgG, 

Ptzticc  ^E(2  Co 

UiiivGrsity  of  Virginis., 

(J  ho-Tlottcsvillc 

l( 

Randolph  Macon  College, 

JSoydtown, 

t( 

University  of  JNorth  Ccirolina., 

Chapel-Hilly 

North  Carolina. 

Davidson  College, 

JSJechlenbuTgf 

College  of  South  Carolina, 

Columbia,, 

South  Carolina. 

University  of  ^labauiQ, 

l^uscaloosa 

At  a  n  a  A/r  a 

jLiiigran'^e  College, 

Ija^van^Cf 

(t 

LiOuisiana  College, 

Jackson 

Louisiana. 

Jefferson  College, 

^Vashin^toTif 

IVIississippi. 

Oakland  College, 

Oakland 

University  of  Tennessee, 

Nashville, 

Tennessee. 

St.  Joseph's  College, 

Bardstown, 

Kentucky. 

Centre  College, 

Danville, 

Augusta  College, 

Augusta, 

ti 

Cumberland  College, 

Pvijiceton, 

Georgetown  College, 

Geo'X'getown, 

Bacon  College, 

Harrodsburghf 

University  of  Ohio, 

Athens, 

Ohio. 

Western  Reserve  College, 

Hudson, 

Oberlin  Institute, 

Oberlin, 

tt 

Miami  University, 

Oxford, 

Franklin  College, 

New  Athens, 

„ 

JC\.tJiiycm  v^uiicgc;. 

Gambier, 

(6 

Granville, 

tt 

v^inciiiiidii  V^UllCgt;, 

Cincinnati, 

^^ 

»VOUUW«lIU  v^UliCgc, 

Cincinnati, 

<t 

Indiana  College, 

Bloomington, 

Indiana 

ooum  ndnovt!!  Vytpm^gc, 

South  Hanover 

VV  dUaisil  V/UUCgC, 

Crauyf ordsville. 

(( 

Tllinr»io  r^rkllporP 
JlllilOlo  v^uiicgc. 

Jacksonville, 

Illinois 

onuriien  ^.>oiiege. 

Upper  A.lt07i, 

McKendroan  College, 

Lebanon 

University  of  St.  Louis, 

St.  Louis, 

Missouri 

St.  Charles  College, 

St.  Charles, 

Michigan  University, 

Ann  Arbour, 

xMlOHIGAM 

The  Military  Academy, 

Georgetown, 

Kentucky. 

Charleston  College, 

Charleston, 

South  Carolina 

11 


.Parker's  Ndtanil  PJiiJosnphy. 


NATURAL  AND  EXPERIMENTAL  PHILOSOPHY 

FOR   SCHOOLS   AND  ACADEMIES, 
BY  R.  G.  PARKER,  A.  M. 

FKlNCirAL  OF  THE  JOHNSON  GRAMMAR  SCHOOL,  BOSTON,  AUTHOR  OF  AIDS 
TO  ENGLISH  COMPOSITION,  ETC.,  E  TC. 

I.  PARKER'S  FIRST  LESSONS  IN  NATURAL  PHILOSOPHY. 
II.  PARKER'S  COMPENDIUM  OF  NATURAL  AND  EXPERIMENTAL 
PHILOSOPHY. 


PARKER'S  FIRST  LESSONS  IN  NATURAL  PHILOSOPHY, 
Embracing  the  Elements  of  the  Science.    Illustrated  with  numerous 
engravings.    Designed  for  young  beginners.    Price  38  cts. 
It  is  the  design  of  this  little  book,  to  present  to  the  minds  of  the 
youth  of  the  country  a  view  of  the  laws  of  Nature — as  they  are 
exhibited  in  the  Natural  World. 

Reading  books  should  be  used  in  schools  for  the  double  object  of 
teaching  the  child  to  read,  and  storing  his  mind  with  pleasant  and 
useful  ideas. 

The  form  of  instruction  by  dialogue,  being  the  simplest,  has 
been  adopted — and  by  means  of  the  simple  question  and  the  ap- 
propriate answer,  a  general  view  of  the  laws  of  the  physical  uni- 
verse has  been  rendered  so  intelligible,  as  to  be  easily  understood 
by  children  who  are  able  to  read  intelligibly. 

Ii  is  confidently  believed  that  this  book  will  form  an  important 
era  m  the  progress  of  common-school  education 

PARKER'S  COMPENDIUM  OF  NATURAL  AND  EXPERIMENTAL 
PHILOSOPHY. 

Embracing  the  Elementary  principles  of  Mechanics,  Hydrostatics ^  Hy- 
draulics, Pneumatics,  Acoustics,  Pyronomics,  Optics,  Astronomy, 
Galvanism,  Magnetism,  Electro-Magnetism,  Magneto-Electricity, 
with  a  description  of  the  Steam  and  Locomotive  Engines.  Illustrated 
by  numerous  diagrams.    Price  $1.00. 

The  use  of  school  apparatus  for  illustrating  and  exemplifying 
the  principles  of  Natural  and  Experimental  Philosophy,  has,  with- 
in the  last  few  years,  become  so  general  as  to  render  necessary  a 
work  which  should  combine,  in  the  same  course  of  instruction,  the 
theory,  with  a  full  description  of  the  apparatus  necessary  for  illus- 
tration and  experiment. 

The  work  of  Professor  Parker,  it  is  confidently  believed,  fully 
meets  that  requirement.    It  is  also  very  full  in  the  general  facts 

(12) 


Parker\'f  Natural  Philosophy. 


which  it  presents — clear  and  concise  in  its  style,  and  entirely 
scientific  and  natural  in  its  arrangement.  The  following  features 
will,  it  is  hoped,  commend  the  work  to  public  favor. 

1.  It  is  adapted  to  the  present  state  of  natural  science  ;  embraces 
a  wider  field,  and  contains  a  greater  amount  of  information  on  the 
respective  subjects  of  which  it  treats,  than  any  other  elementary 
treatise  of  its  size. 

2.  It  contains  an  engraving  of  the  Boston  School  set  of  philo- 
sophical apparatus ;  a  description  of  the  instruments,  and  an  ac- 
count 01  many  experiments  which  can  be  performed  by  means  ol 
the  apparatus. 

3.  It  is  enriched  by  a  representation  and  a  description  of  the 
Locomotive  and  the  Stationary  Steam  Engines,  in  their  latest  and 
most  approved  forms. 

4.  Besides  embracing  a  copious  account  of  the  principles  ot 
Electricity  and  Magnetism,  its  value  is  enhanced  by  the  introduc- 
tion of  the  science  of  Pyronomics,  together  with  the  new  science 
of  Electro-Magnetism  and  Magneto-Electricity. 

5.  It  is  peculiarly  adapted  to  the  convenience  of  study  and  of 
recitation,  by  the  figures  and  diagrams  being  first  placed  side  by 
side  with  the  illustrations,  and  then  repeated  on  separate  leaves  at 
the  end  of  the  volume.  The  number  is  also  given,  where  each 
principle  may  be  found,  to  which  allusion  is  made  throughout  the 
volume. 

6.  It  presents  the  most  important  principles  of  science  in  a 
larger  type  ;  while  the  deductions  from  these  principles,  and  the 
illustrations,  are  contained  in  a  smaller  letter.  Much  useful  and 
interesting  matter  is  also  crowded  into  notes  at  the  bottom  of  the 
page.  By  this  arrangement,  the  pupil  can  never  be  at  a  loss  to 
distinguish  the  parts  of  a  lesson  which  are  of  primary  importance  ; 
nor  will  he  be  in  danger  of  mistaking  theory  and  conjecture  for  fact. 

7.  It  contains  a  number  of  original  illustrations,  which  the  author 
has  found  more  intelligible  to  young  students  than  those  which  he 
has  met  elsewhere. 

8.  Nothing  has  been  omitted  which  is  usually  contained  in  an 
elementary  treatise. 

9.  A  full  description  is  given  of  the  Magnetic  Telegraph,  and  the 
principles  of  its  construction  are  fully  explained. 

10.  For  the  purpose  of  aiding  the  teacher  in  conducting  an  ex- 
amination through  an  entire  subject,  or  indeed,  through  the  whole 
book,  if  necessary,  all  the  diagrams  have  been  repeated  at  the 
end  of  the  work,  and  questions  proposed  on  the  left-hand  page  im- 
mediately opposite.  This  arrangement  will  permit  ihe  pupil  to 
use  the  figure,  in  his  recitation,  if  he  have  not  time  to  make  it  on 
the  black-board,  and  will  also  enable  him  to  review  several  lessons 
and  recall  all  the  principles  by  simply  reading  the  questions,  and 
analyzing  the  diagrams. 


It 


Farhrs  JVa  t  u  ra  I  Ph  ilosoph]/. 


From  the  Wayne  County  Whig. 
After  a  careful  examination  of  Ihis  work,  we  fuid  that  it  is  well  calculated  for 
the  purpose  lor  which  it  is  miciuled,  and  bet.ter  adapted  to  the  state  of  natural 
science  at  the  present  tune,  than  any  other  similar  production  with  which  we 
are  acquainted.  The  design  of  the  author,  in  the  preparation  of  this  work,  was 
to  present  to  the  public  an  elementary  treatise  unencumbered  with  matter  that  is 
not  intnnately  connected  with  this  science,  and  to  give  a  greater  amount  of  in- 
formation on  the  respective  subjects  of  which  it  treats,  than  any  other  school- 
book  of  an  elementary  character.  The  most  remarkable  feature  in  the  style  of 
this  work  is  its  extreme  brevity.  In  the  arrangement  of  the  subject  and  the  man- 
ner of  presenting  it,  there  are  some  peculiarities  which  are,  in  our  opinion,  de- 
cided improvements.  The  more  important  principles  of  this  interesting  science 
are  given  in  a  few  words,  and  with  admirable  perspicuity,  in  a  larger  type  ;  while 
the  deductions  from  these  principles,  and  the  illustrations  are  contained  in  a 
smaller  letter.  Much  useful  and  interesting  matter  is  also  given  in  notes  at  the 
bottom  of  the  page. 

This  volume  is  designed  expressly  to  accompany  the  Boston  School  Set  of  Philo- 
sophical .Apparatus ;  but  the  numerous  diagrams  with  which  it  is  illustrated,  are 
so  well  executed  and  so  easily  understood,  that  the  assistance  of  the  Apparatus 
is  hardly  necessary  to  a  thorough  knowledge  of  the  science.  The  trustees  of  the 
Lyons  Union  School  having  recently  procured  a  complete  set  of  the  above  Ap- 
paratus, this  work  will  now  be  used  as  a  text-book  in  that  institution. 


Leicester  Academy,  April  12,  1848. 

Messrs.  A.  S.  Barnes  &  Co.: 

.Sirs ;— I  have  examined  Parker's  Natural  Thilosophy,  and  am  much  pleased 
with  it.  I  think  I  shall  introduce  it  into  the  academy  the  coming  term.  It  seems 
to  me  to  have  hit  a  happy  medium  between  the  too  simple  and  the  too  abstract. 
The  notes  containing  facts,  and  showing  the  reasons  of  many  things  that  are  of 
common  occurrence  m  every-day  life,  seem  to  me  to  be  a  valuable  feature  of  the 

Very  respectfully,  yours,  B.  A.  SMITH. 


From  the  New  York  Evening  Post. 
Professor  Parker's  book  embraces  the  latest  results  of  investigation  on  the  sub- 
jects of  which  it  treats.  It  has  a  separate  title  for  the  laws  of  heat,  or  Pyronom- 
ics,  which  have  been  lately  added  to  the  list  of  sciences,  as  well  as  electro  mag- 
netism and  magneto  electricity.  The  matter  is  well  arranged,  and  the  style  of 
statement  clear  and  concise.  The  iigures  and  diagrams  are  placed  side  by  side 
with  the  text  they  illustrate,  which  is  greatly  for  the  convenience  of  the  student. 
We  cheerfully  commend  the  book  to  the  favorable  attention  of  the  public. 


From  the  Albany  Spectator. 
Ihis  is  a  school-book  of  no  mean  pretensions  and  of  no  ordinary  value.  It  is 
admirably  adapted  to  the  present  state  of  natural  science  ;  and  besides  contain- 
ing engravings  of  the  Boston  school  set  of  philosophical  apparatus,  embodies 
more  information  on  every  subject  on  which  it  treats  than  any  other  elementary 
work  of  its  size  that  we  have  examined.  It  abounds  with  all  the  necessary  helps 
in  prosecuting  the  study  of  the  science,  and  as  its  value  becomes  known  it  can- 
not fail  to  be  generally  adopted  as  a  text-book. 

14 


Parker's  Xatural  Pkilosoj^hy. 


From  the  Newark  Daily  Advertiser. 
A  work  adapted  to  ihe  present  state  of  natural  science  is  greatly  needed  in  all 
our  schools,  and  the  appearance  of  one  meeting  all  ordinary  wants  must  be  haiied 
with  pleasure  by  those  who  feel  an  interest  in  the  cause  of  education.  Mr.  Par- 
ker  s  work  embraces  a  wider  field,  and  contains  a  greater  amount  of  information 
on  the  respective  subjects  of  which  it  treats,  than  any  other  elementar}-  treatise 
of  its  size,  and  is  rendered  peculiarly  valuable  by  the  introduction  of  the  science  of 
Pyronomics,  together  with  the  new  sciences  of  Electro-Magnetism  and  Magneto 
Electricity.  We  have  seldom  met  w-ith  a  w  ork  so  well  adapted  to  the  conveni- 
ence of  study  and  recitation,  and  regard  as  highly  worthy  of  commendation  the 
care  which  the  author  has  taken  to  prevent  the  pupil  from  mistaking  theory  and 
conjecture  for  fact.  We  predict  for  this  valuable  and  beautifuUy  printed  w 
the  utmost  success. 


From  the  New  York  Courier  and  Enquirer 
"A  School  Compendium  of  Xatural  and  Experimental  Philosophy,"  by  Richard 
Green  Parker,  has  just  been  issued  by  Barnes  &  Co.  Mr.  Parker  has  had  a  good 
deal  of  experience  in  the  business  of  practical  instruction,  and  is,  also,  the  author 
of  w-orks  which  have  been  widely  adopted  in  schools.  The  present  volume  strikes 
us  as  having  very  marked  merit,  and  we  cannot  doubt  it  will  be  well  received. 


4    o  x»  ^  York,  May,  1848. 

Messrs.  A.  S.  Barnes  &  Co.-. 

Gent.  :—l  have  no  hesitation  in  saying  that  Parker's  Natural  Philosophy  is  the 

most  valuable  elementary  work  I  have  seen  :  the  arrangement  of  the  subjects 

and  the  clearness  of  the  definitions  render  it  an  excellent  adjunct  to  a  teacher. 

For  the  last  seven  years  I  have  used  it  in  various  schools  as  a  text-book  for  my 

lectures  on  Natural  Philosophy,  and  am  happy  to  find  that  in  the  new  edition 

much  important  matter  is  added,  more  especially  on  the  subjects  of  Electricity 

and  Electro-Magnetism. 

With  respect,  Gentlemen, 

Your  obedient  servant, 

GILBERT  LANGDON  HUME, 

Teacher  of  Natural  Philosophy  and  Mathematics  in  N.  Y.  city.- 

New  York,  May  2,  184a 
We  have  used  Parker's  Compend  of  Natural  Philosophy  for  many  years,  and 
consider  it  an  excellent  work  on  the  various  topics  of  which  it  treats. 

Yours,  &c.  FORREST  &  McELLIGOTT, 

Principals  of  the  Collegiate  School 

From  the  Lynchburg  Virginian. 
The  volume  before  us  strikes  us  as  containing  more  to  recommend  it  than  any 
one  of  its  class  with  which  we  are  acquainted.  It  is  adapted  to  the  present  state 
of  natural  science  ;  embraces  a  wider  field,  and  contains  a  greater  amount  of  in- 
formation on  the  respective  subjects  of  which  it  treats,  than  any  other  elementary 
treatise  of  its  size.  It  contains  descriptions  of  the  steam-engine,  stationary  and 
locomotive,  and  of  the  magnetic  telegraph.  It  embraces  a  copious  account  of 
the  principles  of  electricity  and  magnetism,  under  all  their  modifications,  and  is 
embellished  by  a  vast  number  of  illustrations  and  diagrams.  There  is  appended 
a  series  of  questions  for  examination,  copious  and  pertinent 

15 


Gillespie  s  Manual  of  Road- Making, 


ROADS   AND  RAILROADS. 

A  MANUAL  OF  ROAD-MAKING: 
Comprisincr  the  principles  and  practice  of  the  Location,  Construe 
tion,  and  Improvement  of  Roads,  (common,  macadam,  paved 
plank,  &c.,)  and  Railroads.     By  VV.  M.  Gillespie,  A.  M., 
Professor  of  Civil  Engineering  in  Union  College.    Price  $1.50. 

Recommendation  from  Professor  Mahan. 
I  have  very  carefully  looked  over  Professor  Gillespie's  Manual  of  Road- 
Makmg.  It  IS,  in  all  respects,  the  best  work  on  this  subject  with  which  I  am  ac- 
quainted ;  being,  from  its  arrangement,  --P^^.^^— ^^^^^^f  Iflel 
adapted  to  the  wants  of  Students  of  Civil  Engineermg,  and  P^^P^f 
sons  in  any  way  engaged  in  the  construction  or  supervision  of  roads.  The  ap- 
pearance of  suL  a  work,  twenty  years  earlier,  would  have  been  a  truly  national 
b™and  it  is  to  be  hoped  that  its  introduction  into  our  seminaries  may  be  so 
general  as  to  make  a  knowledge  of  the  prmciples  and  practice  of  this  branch  of 
engineering,  as  popular  as  is  its  importance  to  all  classes  of  the  community. 

^Signed,)  Q  MAHAN, 

Professor  of  Civil  Engineering  in  the  Military  ) 
Academy  of  the  United  States.  ) 

From  a  Report  of  a  Committee  of  the  American  Institute. 
This  work  contains  in  a  condensed  form,  all  the  principles,  both  ^^^^^f ^^^^^^ 
modern,  of  this  most  important  art ;  and  almost  every  thing  useful  in  the  great 

mass  of  writers  on  this  subject  Such  a  work  as  this  P^"  f^^J^^ 

service  for  those  who  are  destined  to  construct  roads-by  showmgnot  only  what 
ought  to  be  done,  but  what  ought  not  to  be  done  ;  thus  saving  immense  outlay  of 

money,  and  loss  of  time  in  experiments  The  committee  therefore,  recom- 

mend  it  to  the  public. 

From  the  American  Railroad  Journal. 
The  views  of  the  author  are  sound  and  practical,  and  should  be  read  by  the 
people  throughout  the  entire  length  and  breadth  of  the  land.  .  .  .  We  recom- 
mend  this  Manual  to  the  perusal  of  every  tax-payer  for  road-makmg  and  to  the 
young  men  of  the  country,  as  they  will  find  useful  information  m  relation  to  each 
department  of  road-making,  which  will  surely  be  useful  to  them  m  after-life. 

From  Silliman's  American  Journal  of  Science. 
If  the  well-established  principles  of  Road-Making,  which  are  so  plainly  set 
forth  in  Prof.  Gillespie's  valuable  work,  and  so  well  illustrated,  could  be  once 
put  into  general  use  in  this  country,  every  traveller  would  bear  testimony  to  the 
fact  that  the  author  is  a  great  public  benefactor. 

From  the  Journal  of  the  Franklin  Institute. 
This  small  volume  contains  much  valuable  matter,  derived  from  the  best 
authorities,  and  set  forth  in  a  clear  and  simple  style.   For  the  want  of  mforma- 
tion  which  is  contained  in  this  Manual,  serious  mistakes  are  frequently  made, 
a:,a  roads  are  badlv  located  and  badly  constructed  by  persons  ignorant  of  the  true 

(]<>) 


Gillespie's  Manual  of  Road- Making, 


grinciples  which  ought  to  govern  in  such  cases.  By  the  extensive  circulation  of 
such  books  as  that  now  before  us,  and  the  imparting  of  sound  views  on  the  sub- 
ject to  the  students  of  our  collegiate  institutions,  we  may  hope  for  a  change  for 
.he  better  in  this  respect. 

From  the  Albany  Cultivator. 
The  author  of  this  work  has  supplied  a  desideratum  which  has  long  existed. 
Perhaps  there  is  no  subject  on  which  information  is  more  needed  by  the  country 
in  general  than  that  of  Road-Making.  Prof.  Gillespie  has  taken  up  the  subject 
in  a  proper  manner,  beginning  the  work  at  the  right  place,  and  prosecuting  it  in 
systematic  order  to'its  completion. 

From  the  New  York  Tribune, 
It  would  astonish  many  path-masters"  to  see  how  much  they  don't  know 
with  regard  to  the  very  business  they  have  considered  themselves  such  adepts  in. 
Yet  all  is  so  simple,  so  lucid,  so  straight-forward,  so  manifestly  true,  that  the 
most  ordinary  and  least  instructed  mind  cannot  fail  to  profit  by  it.  We  trust  this 
useful  and  excellent  volume  may  find  its  way  into  every  village  library  if  not 
into  every  school  library,  as  well  as  into  the  hands  of  every  man  interested  in 
road-making.  Its  illustrations  are  very  plain  and  valuable,  and  we  cannot  doubt 
th^at  the  work  will  be  a  welcome  visiter  in  many  a  neighborhood,  and  that  bad 
roads  will  vanish  before  it. 

From  the  Neioark  Daily  Advertiser. 
This  elaborate  and  admirable  work  combines  in  a  systematic  and  symmetrical 
form  the  results  of  an  engineering  experience  in  all  parts  of  the  Union,  and  of  an 
examination  of  the  great  roads  of  Europe,  with  a  careful  digestion  of  all  acces- 
sible authorities.  The  six  chapters  into  which  it  is  divided  comprehend  a 
methodical  treatise  upon  every  part  of  the  whole  subject ;  showing  what  roads 
ought  to  be  in  the  vital  points  of  direction,  slopes,  shape,  surface,  and  cost,  and 
giving  methods  of  performing  all  the  necessary  measurements  of  distances,  di- 
rections, and  heights,  without  the  use  of  any  instruments  but  such  as  any 
mechanic  can  make,  and  any  farmer  use.  Bridges,  Railroads,  and  City  Streets 
are  also  treated  of  at  length  and  with  good  sense. 

From  the  Vermont  Chronicle. 
To  selectmen  and  others  who  may  have  any  thing  to  do  with  these  improve- 
ments, we  would  earnestly  recommend  the  book  named  above.  The  author  is  a 
man  of  science,  (Professor  of  Civil  Engineering  at  Union  College,)  and  his  work 
embraces  a  full  discussion  of  both  the  principles  and  practice  of  Road- Making 
A  little  study  of  this  work  may  often  lead  to  results  of  importance  to  whole  towns 
and  counties. 

From  the  Home  Journal. 
The  author  of  this  book  holds  a  quill  so  skilful  and  dair^r  in  light  literature, 
that  we  were  not  prepared  with  laurels  to  crown  him  for  a  scientific  work  ;  but 
we  see,  by  the  learned  critics,  that  this  fruit  of  his  study  of  his  profession  as  an 
engineer,  is  very  worthy  of  high  commendation,  and  a  valuable  addition  to  the 
useful  literature  of  the  day. 

(17) 


WillanVs  Series  of  School  Histories  and  Charts, 


MRS.  EMMA  WILLARD'S 
SERIES  OP  SCHOOL  HISTORIES  AND  CHARTS. 

I.  WILLARD'S   HISTORY  OF  THE  UNITED  STATES,  OR  RE- 
PUBLIC OF  AMERICA,  8vo.    Price  $1.50. 
11.  WILLARD-S  SCHOOL  HISTORY  OF  THE  UNITED  STATES. 
111.  WILLARD^S  AMERICAN  CHRONOGRAPHER,  il.OO. 
A  Chart  of  American  History. 


I.  WILLARD'S  UNIVERSAL  HISTORY  IN  PERSPECTIVE.  $1.50. 
II.  WILLaRD'S  temple  of  time,  $1.25. 

A  Chart  of  Universal  History 


W  I  L  L  A  R  D'S 
HISTORY  OF  THE   UNITED  STATES. 


The  large  work  is  designed  as  a  Text-Book  for  Academies  and 
Female  Seminaries:  and  also  for  District  School  and  Family 
Libraries.  The  small  work  being  an  Abridgment  of  the  same,  is 
designed  as  a  Text-Book  for  Common  Schools,  The  originality 
of  the  plan  consists  in  dividing  the  time  into  periods,  of  which 
the  beginnings  and  terminations  are  marked  by  important  events ; 
and  constructing  a  series  of  maps  illustrating  the  progress  of  the 
settlement  of  the  country,  and  the  regular  advances  of  civilization. 
The  Chronographic  Chart,  gives  by  simple  inspection,  a  view  of 
the  divisions  of  the  work,  and  the  events  which  mark  the  be- 
ginning  and  termination  of  each  period  into  which  it  is  divided, 
A  full  chronological  table  will  be  found,  in  which  all  the  events  ot 
the  History  are  arranged  in  the  order  of  time.  There  is  appended 
to  the  work  the  Constitution  of  the  United  States,  and  a  series  ot 
questions  adapted  to  each  chapter,  so  that  the  work  may  be  used 
in  schools  and  for  private  instruction. 

The  Hon.  Daniel  Webster  says,  of  an  early  edition  of  the  above  work,  in  a  letter 
to  the  author,  "  I  keep  it  near  me,  as  a  Book  of  Reference,  accurate  in  facts  and  dates  J' 

(18; 


WillarcVs  Series  of  School  Histories  and  Charts. 


W  I  LL  A  RD'S 
AMERICAN   CH  RONOGRAPH  ER, 

DESIGNED   TO   ACCOMPANY  WILLARd's  HISTORY  OF 
THE   UNITED  STATES, 


To  measure  time  by  space  is  universal  among  civilized  nations, 
and  as  the  hours,  and  minutes,  and  seconds  of  a  clock  measure  the 
time  of  a  day,  so  do  the  centuries,  tens,  and  single  years  of  this 
Chronographer,  measure  the  time  of  American  History.  A 
general  knowledge  of  chronology  is  as  indispensable  to  history,  as 
a  general  knowledge  of  latitude  and  longitude  is  to  geography. 
But  to  learn  single  dates,  apart  from  a  general  plan  of  chronology 
addressed  to  the  eye,  is  as  useless  as  to  learn  latitudes  and  longi- 
tudes without  reference  to  a  map.  The  eye  is  the  only  medium 
of  permanent  impression.  The  essential  point  in  a  date,  is  to 
know  the  relative  place  of  an  event,  or  how  it  stands  in  time  com- 
pared with  other  important  events.  The  scholar  in  the  school- 
loom,  or  the  gentleman  in  his  study,  wants  such  a  visible  plan  of 
time  for  the  study  of  history,  the  same  as  he  wants  the  visible 
plan  of  space,  viz.,  a  map  for  the  study  of  geography,  or  of  books 
of  travels.  Such  is  the  object  of  Willard's  Chronographer  of 
American  History. 

Extract  from  a  Report  of  the  Ward  School  Teachers^  Association 
of  the  City  of  New  York. 
The  Committee  on  Books  of  the  Ward  School  Association  respectfully  report :  ■ 
That  they  have  exammed  Mrs.  Willard's  History  of  the  United  States  with 
peculiar  interest,  and  are  free  to  say.  that  it  is  in  their  opinion  decidedly  the  best 
treatise  on  this  interesting  subject  that  they  have  seen.   *  * 

As  a  school-book,  its  proper  place  is  among  the  first.  The  language  is  remark- 
able for  simplicity,  perspicuity,  and  neatness  ;  youth  could  not  be  trained  to  a 
better  taste  for  language  than  this  is  calculated  to  impart.  The  history  is  so 
written  as  to  lead  to  geographical  examinations,  and  impresses  by  practice  the 
habit  to  read  history  with  maps.  It  places  at  once,  in  the  hands  of  American 
youth,  the  history  of  their  country  from  the  day  of  its  discovery  to  tne  present 
time,  and  exhibits  a  clear  arrangement  of  ail  the  great  and  good  deeds  of  their 
ancestors,  of  which  they  now  enjoy  the  benefits,  and  inherit  the  renown.  The 
struggles,  sufferings,  firmness,  and  piety  of  the  first  settlers  are  delineated  with  a 
masterly  hand. 

The  gradual  enlargement  of  our  dominions,  and  the  development  of  our  na- 
tional energies,  are  traced  with  a  minute  accuracy,  which  the  general  plan  of  the 
work  indicates. 

The  events  and  achievements  of  the  Revolution  and  of  the  last  war,  are 
brought  out  in  a  clear  light,  and  the  subsequent  history' of  our  national  policy 
and  advancement  strikingly  portrayed,  without  being  disfigured  b\  that  tinge 

■(19) 


WilJard's  Series  of  School  Histories  and  Charts, 


of  party  buis  which  is  so  dillicult  to  be  guarded  against  by  historians  of  their  own 

^^''Hu^  details  of  tiie  discovery  of  this  continent  by  Columbus,  and  of  the  early 
settlements  by  tlve  Spaniards,  Portuguese,  and  other  European  nations,  are  all  oi 
essential  interest  to  the  student  of  American  history,  and  will  be  found  sufficiently 
minute  to  render  the  history  of  the  continent  full  and  complete.  The  diflTerent 
periods  of  time,  together  with  the  particular  dates,  are  distinctly  set  forth  with 
statistical  notes  on  the  margin  of  each  page,-and  these  afford  much  information 
w  ithout  perusing  the  pages. 

The  maps  are  beautifully  executed,  with  the  locality  of  places  where  particular 
events  occurred,  and  the  surrounding  country  particularly  delineated.  These 
are  admirably  calculated  to  make  lasting  impressions  on  the  mind. 

The  day  has  now  arrived  when  every  child  should  be  acquainted  with  the  his- 
tory o{  his  country  ;  and  your  Committee  rejoice  that  a  work  so  full  and  clear  can 
be  placed  within  the  reach  of  every  one. 

The  atudent  will  learn,  by  reading  a  few  pages,  how  much  reason  he  has  to  be 
proud  of  his  country-of  its  institutions-of  its  founders-of  its  heroes  and  states- 
men :  and  by  such  lessons  are  we  not  to  hope  that  those  who  come  after  us  will 
be  instructed  in  their  duties  as  citizens,  and  their  obligations  as  patriots  1 

Your  Committee  are  anxious  to  see  this  work  extensively  used  in  aU  the  schools 
in  the  United  States. 

(Signed,) 

SENECA  DURAND, 
EDWARD  McELROY, 
JOHN  WALSH. 

The  Committee  would  respectfully  offer  the  following  resolution  : 
Resolved,  That  Mrs.  Emma  Willard's  History  of  the  United  States  be  adopted 
by  this  Association,  and  its  introduction  into  our  schools  earnestly  recom- 
mended. 

At  a  meeting  of  the  Board  of  the  Ward  School  Teacners'  Association,  January 
20th,  1847,  the  above  Resolution  was  adopted.— (Copied  from  the  Minutes.) 

From  the  Boston  Traveller. 

We  consider  the  work  a  remarkable  one,  in  that  it  forms  the  best  book  for 
general  reading  and  reference  published,  and  at  the  same  time  has  no  equal,  in 
our  opinion,  as  a  text-book.  On  this  latter  point,  the  profession  which  its  author 
has  so  long  followed  with  such  signal  success,  rendered  her  peculiarly  a  fitting 
person  to  prepare  a  text-book.  None  but  a  practical  teacher  is  capable  of  pre- 
paring a  good  school-book  ;  and  as  woman  has  so  much  to  do  in  forming  our 
3arly "character,  why  should  her  influence  cease  at  the  fireside— why  not  en- 
courage her  to  exert  her  talents  still,  in  preparing  school  and  other  books  for 
after  years  ?   No  hand  can  do  it  better. 

The  typography  of  this  work  is  altogether  in  good  taste. 

From  the  Cincinnati  Gazette. 
Mrs.  Willard's  School  History  of  the  United  States.— It  is  one  of  those 
rare  things,  a  good  school-book  ;  infinitely  better  than  any  of  the  United  States 
Histories  fitted  for  schools,  which  we  have  at  present.  It  is  quite  full  enough, 
and  yet  condensed  with  great  care  and  skill.  The  style  is  clear  and  simple- 
Mrs  Willard  having  avoided  those  immense  Johnsonian  words  which  Grimshaw 
and  other  writers  for  children  love  to  put  into  their  works,  while,  at  the  same 
time  there  is  nothing  of  the  pap  style  about  it.   The  arrangement  is  excellent, 

(20) 


Willard^s  Series  of  School  Histories  and  Charts. 


the  chapters  of  a  good  length  ;  every  page  is  dated,  and  a  marginal  index  makes 
reference  easy.  But  the  best  feature  in  the  work  is  its  series  of  maps  ;  we  have 
the  country  as  it  was  when  filled  with  Indians  ;  as  granted  to  Gilbert ;  as  di- 
vided at  the  time  the  Pilgrims  came  over  ;  as  apportioned  in  1643 ;  the  West 
while  in  possession  of  France  ;  the  Atlantic  coast  in  1733  ;  in  1763 ;  as  in  the 
Revolution,  with  the  position  of  the  army  at  various  points  ;  at  the  close  of  the 
Revolutionary  War;  during  the  war  of  1812-15;  and  in  1840 making  eleven 
most  excellent  maps,  such  as  every  school  history  should  have.  When  we 
think  of  the  unintelligible,  incomplete,  badly  written,  badly  arranged,  worthless 
work  of  Grimshaw  which  has  been  so  long  used  in  our  schools,  we  feel  that 
every  scholar  and  teacher  owes  a  debt  of  gratitude  to  Mrs.  Willard.  Miiss 
Robins  has  done  for  English  History,  what  Mrs.  Willard  has  now  done  for 
American,  and  we  trust  these  two  works  will  be  followed  by  others  of  as  high  or 
higher  character.  We  recommend  Mrs.  Willard's  work  as  better  than  any  we 
know  of  on  the  same  subject ;  not  excepting  Bancroft's  abridgment.  This  work, 
followed  by  the  careful  reading  of  Mr.  Bancroft's  full  work,  is  all  that  would  be 
needed  up  to  the  point  where  Bancroft  stops  ;  from  that  point,  Pitkin  and  Mar- 
shall imperfectly  supply  the  place,  which  Bancroft  and  Sparks  will  soon  fill. 

From  the  United  States  Gazette. 

Mrs.  Willard  is  well  known  throughout  the  country  as  a  lady  of  high  attain 
ments,  who  has  distinguished  herself  as  the  Principal  of  Female  Academies,  that 
have  sent  abroad  some  of  the  most  accomplished  females  of  the  land. 

The  plan  of  the  authoress  is  to  divide  the  time  into  periods,  of  which  the  be- 
gmning  and  the  end  are  marked  by  some  important  event,  and  then  care  has 
been  taken  to  make  plain  the  events  of  intermediate  periods.  The  style  is  clear, 
and  there  appears  no  confusion  in  the  narrative.  In  looking  through  the  work, 
we  do  not  discover  that  the  author  has  any  early  prejudices  to  gratify.  The 
book,  therefore,  so  far  as  we  have  been  able  to  judge,  may  be  safely  recom- 
mended as  one  of  great  merit,  and  the  maps  and  marginal  notes,  and  series  of 
questions,  give  additional  value  to  the  work. 

From  the  Ntwhuryp&rt  Watchman. 
An  Abridged  History  of  the  United  States:  By  Emma  Willard. — We 

think  we  are  warranted  in  saying,  that  it  is  better  adapted  to  meet  the  wants  of 
our  schools  and  academies  in  which  history  is  pursued,  than  any  other  work  of 
the  kind  now  before  the  public. 

The  style  is  perspicuous  and  flowing,  and  the  prominent  points  of  our  history  are 
presented  in  such  a  manner  as  to  make  a  deep  and  lasting  impression  on  the  mind. 

We  could  conscientiously  say  much  more  in  praise  of  this  book,  but  must  content 
ourselves  by  heartily  commending  it  to  the  attention  of  those  who  are  anxious 
to  find  a  good  text-book  of  American  history  for  the  use  of  schools. 

From  the  Albany  Evening  Journal. 
Willard's  United  States.— This  work  is  well  printed  on  strong  white  paper, 
and  is  bound  in  a  plain  substantial  manner — all-important  requisites  in  a  school- 
book.  The  text  is  prepared  with  equal  skill  and  judgment.  The  memory  of  the 
youthful  student  is  aided  by  a  number  of  spirited  illustrations — by  no  means  un- 
important auxiliaries— while  to  lighten  the  labors  of  the  teacher,  a  series  of  ques- 
tions is  adapted  to  each  chapter.  Nor  is  its  usefulness  limited  to  the  school-roora 
As  a  book  of  reference  for  editors,  lawyers,  politicians,  and  others,  where  dates  and 
facts  connected  ^ath  every  important  event  in  American  History  may  be  readily 
found,  this  little  book  is  truly  valuable. 

21 


Willard's  Scrim  of  School  Histories  and  Charts. 


W  I  LL  A  R  D'S 
UNIVERSAL  HISTORY  IN  PERSPECTIVE. 

ILLUSTRATED  WITJI   MAPS  AND  ENGRAVINGS. 


THIS  WORK  IS  ARRANGED  IN  THREE  PARTS,  VIZI 

ANCIENT,  MIDDLE,  AND  MODERN  HISTORY. 

1.  Ancient  History  is  divided  into  six  periods — comprising 
events  from  the  Creation,  to  the  Birth  of  our  Saviour. 

2.  Middle  History,  into  five  periods, — from  the  Christian  Era, 
to  the  Discovery  of  America. 

3.  Modern  History,  into  nine  periods, — from  the  Discovery  of 
America,  to  the  present  time.  Each  period  marked  by  some  im- 
portant event  and  illustrated  by  maps  or  engravings. 

The  following  resolution  was  offered  and  adopted  at  a  meeting  of  the  Ward 
School  Teachers'  Association  of  the  City  of  New  York,  January  20th,  1847. 

Resolved,  That  the  Ward  School  Teachers'  Association  of  New  York  con- 
siders Willard's  Universal  History  as  a  book  essentially  adapted  to  the  higher 
classes  of  schools  on  account  of  its  vivacity,  lucidness,  and  intelligent  mode  of 
arrangement,  of  dates  and  questions,  and  that  such  a  work  has  long  been  wanted, 
and  as  such  will  endeavor  to  introduce  it  into  their  respective  schools,  aud 
warmly  recommend  it  to  public  patronage. 


Extract  of  a  Letter  from  Mr.  Elbridge  Smith,  late  Principal  of  the  English 
High  School  of  Worcester,  Mass. 
I  have  recently  introduced  "  Willard's  Universal  History  in  Perspective,"  into 
the  school  under  my  care.   I  am  much  pleased  with  it,  and  think  it  superior  to 
any  other  work  of  the  kind. 

(Signed,) 

ELBRIDGE  SMITH. 

Worcester f  June  5,  1847. 

From  Professor  Charles  B.  Haddock  of  Dartmouth  College^  and  School  Commissioner 
of  the  State  of  New  Hampshire. 
I  am  acquainted  with  Mrs.  Willard's  Histories,  and  entertain  a  high  opinion  of 
them.   They  are  happily  executed,  and  worthy  of  the  long  experience  and  emi- 
nent character  of  their  author. 

(Signed,) 

CHARLES  B.  HADDOCK. 

Dartmouth  College^  Hanover,  Dec.  11,  1846. 

22 


Willard's  Series  of  School  Hhtories  and  Charts. 


W  I  LL  A  RD'S 

TEMPLE   OF  TIME, 

DESIGNED  TO  ACCOMPANY   WILLARd's  UNIVERSAL  HISTORY 


This  Temple  exhibits  at  one  view  the  whole  scheme  of  Uni 
versal  Chronology,  from  the  Creation  to  the  present  time.  Each 
pillar  represents  the  century  corresponding  to  the  number  at  its 
6ase.  The  pillars  are  in  groups  of  tens,  four  groups  before 
Christ,  and  two  after,  the  last  thousand  years  being  deficient  by  a 
part  of  the  nineteenth  and  the  whole  of  the  twentieth  century. 
As  pillars  in  building  are  begun  at  the  bottom,  so  the  time  of  the 
century  represented  by  each  pillar,  is  reckoned  upwards.  (See 
pillar  for  the  eighteenth  century.) 

The  names  on  the  pillars  are  of  those  sovereigns  by  which  the 
age  is  chiefly  distinguished.  The  floor-work  shows  what  have 
been  the  principal  nations  of  the  world,  through  the  several  cen- 
turies, which  may  be  known  by  tracing  to  the  bases  of  the  pil- 
lars on  each  side.  Of  the  principal  nations  of  Europe,  the 
names  of  all  the  sovereigns  now  reigning,  and  of  those  who  have 
reigned  since  the  discovery  of  America,  are  inserted  ;  but  ante- 
cedent to  that  period,  only  the  names  of  the  principal  sovereigns 
are  set  down. 

The  roof  of  the  Temple  contains,  in  five  compartments,  the  names 
of  the  most  celebrated  persons  of  the  age  to  which  they  be- 
longed. The  Temple,  in  so  far  as  the  pillars  and  the  roof  are 
concerned,  might  be  called  the  Temple  of  Time  and  of  Fame. 
All  the  names  inserted  on  those  parts  are  of  persons  not  nowlivino-. 
Along  the  right  margin  of  the  floor-work  and  next  the  base  of 
the  pillars,  are  set  down  some  of  the  most  important  battles.  On 
the  left  corresponding  margin,  are  placed  the  epochs  of  Willard's 
Universal  History.  They  are  selected  with  care,  as  the  best  by 
which  to  divide  this  great  subject.  This  brings  the  Temple  of 
Time  into  closer  connection  with  Willard's  History  than  with  any 
other;  but  it  may  accompany  any  system  of  Universal  History; 
or  it  may  be  used  to  advantage  by  itself,  with  the  aid  of  a  Dic- 
tionary of  Universal  Biography. 

23 


Willard's  Series  of  School  Histories  and  Charts. 


Arqyle  Academy,  J;jn7  12,  1848. 

Messrs.  A.  S.  Haunes  &  Co.: 

6Vri/.;— Allcr  the  second  perusal  of  Willard's  Universal  History  I  have  pro- 
nounced It  the  L)est  work  that  I  have  seen  on  this  subject.  It  should  be  immedi- 
ately introduced  into  all  our  high  schools. 

Respectfully  yours,  D.  A.  IIARSHA,  Principal. 

CORTLANDVILLE  AcADEMY,  NoV.  9 

Mrs.  \Vilhird\s  Universal  History  has  been  a  standard  book  with  us  for  more 
than  a  year,  and  we  have  no  wish  to  part  with  it.  Her  history  of  the  United 
States  w'e  esteem  as  a  dear  friend.    No  one  can  know  that  book  and  not  admire  it. 

W.  C.  LIVINGSTON,  Principal 

Phillips  Academy,  Exeter,  N.  H.,  Aug.  24,  1847. 
Messrs.  A   S.  Barnes  &  Co.: 

a,nt.:--My  estimation  of  Willard's  Histories  may  be  inferred  from  the  fact 
that  Ihey  are  adopted  as  our  text-books  in  this  academy. 

Respectfully  yours,  S.  G.  HOYT. 

Extract  of  a  Letter  to  the  Authoress  from  the  Rev.  John  Lord,  the  celebrated  .American 
Lecturer  on  the  Middle  Ages. 
Having  critically  and  carefully  examined  Mrs.  Willard's  Chronological  Picture 
of  Nations,  I  can  most  cordially  say,  that  in  my  opinion,  it  is  accurate,  original, 
and  comprehensive.  I  question  whether  there  is  any  historical  chart  of  the  kind  in 
existence,  more  valuable  to  historical  students  of  any  age  or  attainment,  or  cal- 
culated to  be  so  useful  in  Literary  Institutions. 

(Signed,)  JOHN  LORD. 

Hartford,  May  18,  1843. 

From  Jona.  Tarbell,  Esq.,  late  Superintendent  of  Essex  County,  New  York. 
Mrs.  Willard  : 

Permit  me  to  say  to  you  directly,  that  I  have  once  carefully  examined  your 
Histories,  and  often  referred  to  their  contents  :  the  conclusion  to  which  I  have 
come  is  the  following,  viz. : 

1st.  The  style  combines  grace,  beauty,  and  strength. 

2d.  The  arrangement  is  new,  and  not  to  be  excelled. 

3d.  They  meet  the  wants  of  our  common  schools. 

4th.  They  are  just  what  every  one  needs,  of  whatever  occupation  or  profes- 
sion, not  only  as  books  of  reference,  but  as  containing  the  whole  of  History,  m  a 
style  of  rare  attainment.  Personally  I  am  an  entire  stranger  to  you,  and  though 
the  most  humble  am  not  the  least  devoted  admirer  of  your  productions. 

Yours,  very  respectfully,  JONATHAN  TARBELL 

24 


Fulton  dc  Eastmans  Principles  of  Penmanship. 


FULTON  &L  EASTMAN'S  PENMANSHIP, 

Illustrated  and  expeditiously  taught  by  the  use  of  a  series  of  Chirographic 
Charts,  a  Key,  emd  a  set  of  School  Writing-Books,  appropriately  ruled. 
I. 

CHmOGRAPmC  CHARTS, 
IN  TVTG  NXMBERS.    (Price  5.00.) 
Chart  >o.  1,  Embraces  Prlmary  Exercises,  and  Elementary  Prlnciples 
IN  Writing. 

Cliart  No.  2,  Embraces  Elementary  Principles  for  Capitals  Combined, 
and  Elementary  Principles  for  Small  Letters  Com- 

BIXED. 

II. 

KEY  TO  CHIROGR.\PHIC  CHARTS; 
Containing  directions  for  the  position  at  the  desk,  and  manner  of  holding 
the  P^n. — Also  for  the  exact  forms  and  proportions  of  letters,  with  Rules 
for  their  execution.    (Price  25  cents.) 

HI. 

SCHOOL  ^^^lITIXG-BOOKS. 

IN  FOUR  NUMBERS.    (Price  12^  cents  each.) 


From  the  Trustees  of  the  Union  School,  Lyons.  X.  Y. 

The  undersigned,  trustees  of  the  Union  District  School  of  the  town  of  Lyons, 
take  this  method  of  expressing  th^ir  approval  of  ''Fulton's  Principles  of  Pen- 
manship." Tliey  have  seen  the  system  m  operation,  durmg  the  past  year,  in  the 
school  with  which  they  are  comiected,  and  are  fully  satisfied  of  its  great  superi- 
ority over  all  other  systems  heretofore  used.  The  '*  Chirographic  Charts."  upon 
which  are  dra\\-n  in  large  size  the  different  letters  and  parts  of  the  letters  of  the 
alphabet,  proportioned  in  accordance  \\-ith  the  rules  laid  down  by  the  author  for 
the  formation  of  each  letter,  and  which,  when  suspended,  cem  be  seen  from  all 
parts  of  a  school-room  of  ordmary  size,  they  regard  as  an  especial  improvement 
upon,  and  advantage  over,  other  modes  of  teachmg  this  art.  "^Mule  the  labor  of 
the  teacher  is  by  this  means  hghtened  a  hundredfold,  from  the  fact  that  the  direc- 
tions and  rules  thus  illustrated,  can  be  explained  to  a  whole  class  at  once,  the 
benefit  to  the  scholar  is  proportionally  increased.  The  charts  being  made  the 
property  of  the  district,  a  uniformity  is  est<ibhshed  in  this  branch  of  instruction, 
and  the  continual  changes  in  books  and  methods  of  teacking,  which  have  hereto- 
fore given  occasion  to  so  much  just  complaint  on  the  part  of  parents  and  guard- 
ians, and  which  have  been  so  prejudicial  to  the  pupil,  are  entirely  avoided.  ^ 

The  brief  space  necessarily  allotted  to  a  notice  of  this  kind,  will  not  permit  the 
undersigned  to  say  all  they  might  say  with  truth  in  praise  of  Mr.  F.'s  system  of 
instruction.  They  therefore  conclude  with  the  remark  that  it  meets  their  entire 
approbation,  and  they  cordially  commend  it  to  the  favorable  notice  of  the  friends 
of  education  generally,  and  would  recommend  it5  adoption  by  academies  and 
common  schools  in  this  and  in  other  states. 

A.  L.  BEAUMOXT 
ELI  JOHXSOX, 

DtHtd  Lyons,  N.  Y.,  April  5th,  lSi7.  DE  WITT  PARSHALL 

25 


Fulton  d'  Fastmtms  Principles  of  Penmanship. 


Newark,  March  3,  1848. 

I  have  examined  w  ith  mucli  care  Fulton's  System  of  I*enmanship,  lately  pub- 
lished by  Messrs.  A.  S.  Barnes  ^  Co.,  of  New  York.  My  attention  has  been  called 
to  the  subject  of  teaclung  penmanship  in  our  public  schools,  from  the  very  mani- 
fest want  of  any  system  that  seemed  at  all  suited  to  the  character  of  our  Ward 
Schools.  Mr.  Fulton's  system  I  deem  to  be  the  best  I  ever  saw,  and  I  have  no 
hesitation  in  recommending  it.  There  is  an  exactness  about  Mr.  F.'s  method  of 
teaching  this  art,  which  seems  to  defy  the  possibility  of  pupils  becoming  any 
thing  but  accomplished  proficients. 

I  have  taken  means  to  procure  the  introduction  of  one  set  of  the  charts  and  a 
number  of  the  copy-books  in  our  schools  as  an  experiment,  and  so  well  satisfied 
am  I  that  the  system  is  what  we  need,  that  I  shall  use  early  measures  to  have 
them  introduced  more  extensively. 

Yours,  &c.,  JNO.  WHITEHEAD, 

Commissioner  of  Public  Schools  for  the  city  of  Newark 

From  the  Superintendent  of  Monroe  County,  West  District. 
Mr.  Levi  S.  Fulton: 

Dear  Sir  ;— I  am  well  pleased  with  the  examination  of  your  series  of  "  Chiro- 
graphic Charts,  for  the  purpose  of  illustrating  and  teaching  the  principles  of  Pen- 
niaiiship."  One  of  the  greatest  obstacles  in  the  way  of  the  scholar's  improve- 
ment in  our  schools,  is  the  frequent  change  of  teachers.  Under  the  instruction 
of  every  new  teacher,  the  scholar  commences  to  learn  anew  hand,  by  attempting 
to  copy  that  of  the  teacher :  the  consequence  is,  that  he  rarely  obtains  a  good 
permanent  hand.  His  efforts  so  often  failing  of  success,  he  becomes  discouraged, 
i^nd  ready  to  abandon  the  exercise  as  a  vexatious  and  hopeless  task. 

By  the  use  of  your  charts,  applying  the  principles  as  taught  in  your  book,  the 
teacher  and  pupil  will  be  very  much  aided  in  the  exercise  ;  the  teacher  illustra- 
ting the''t»rinciples  from  the  chart,  and  the  pupil  practising  upon  them. 

I  rejoice  that  you  have  so  arranged  these  principles,  that  the  art  of  good  pen- 
manship will  be  placed  within  the  reach  of  all  who  desire  to  attain  this  necessary 
accomplishment,  and  I  will  indulge  the  hope,  that  your  works  may  obtain  that 
extensive  circulation  which  their  merits  so  richly  deserve. 

Desiring  your  best  success  in  this  praiseworthy  undertaking,  I  shall  ever  re- 
main your  most  obedient  and  humble  servant, 

JULIUS  A.  PERKINS, 
County  Superintendent,  Monroe  co..  West  District. 

Spencerport,  Dec.  26,  1846. 

Levi  S.  Fulton,  Esq.  : 

Dear  Sir  .-—Your  theory  and  practice  of  Penmanship,  which  I  have  had  several 
opportunities  to  see  tested  and  applied,  is,  in  my  opinion,  truly  philosophical,  and 
fully  justifies  the  high  estimate  formed  of  it  by  all  to  whom  it  has  been  exhibited. 

I  have  examined  the  plan  of  your  proposed  publication,  and  entirely  approve 
of  it.  It  seems  to  me  that  such  a  work  is  greatly  needed,  and  that  its  adoption  as 
a  text- book  would  greatly  facilitate  the  acquisition  of  a  beautiful  but  hitherto  vex- 
atious branch  of  education. 

REV.  0.  R.  HOWARD,  A.  M., 
(Late)  Principal  of  Fairfield  Academy. 

Lyons,  Dec  1,  1846. 

Penfield,  Jan.  31,  1848, 

Dear  Sirs ;— It  is  witn  pleasure  1  inform  you  that  your  Chirographic  Charts  are 
in  use  in  the  Union  School  of  this  village,  with  admirable  success.   Serious  diffi- 

26 


Fulton     Eastman's  Principles  of  Penmanship, 


culties  which  presented  themselves  to  the  learner  of  writing  by  the  old  system 
(imitation  merely)  are  entirely  overcome. 

The  fixed  rules  for  the  formation  of  each  principle  separately,  and  most  espe- 
cially the-general  arrangement  of  your  Charts,  are  desiderata  hitherto  unreached 
Dy  any  system  of  penmanship  with  which  I  am  acquainted. 

In  my  humble  opinion,  they  must  meet  with  universal  approbation. 

Very  respectfully,  yours,  &c.,       '  WM.  D.  SHUART. 

Messrs.  Fulton  &  Eastman  : 

Dear  Str5 ;— I  have  carefully  examined  your  Chirographic  Charts  and  Key,  and 
am  pleased  to  find  your  system  of  penmanship  one  that  at  once  recommends  it- 
self by  its  perfection  and  simplicity— requisites  indispensable  to  successful  ap- 
plication in  our  schools,  and  for  a  lack  of  which,  others  have  proved  failures. 
As  teachers  themselves  are  not  unfrequently  inferior  penmen,  and  have  gained 
their  own  knowledge  of  this  branch  of  education  by  a  random  practice,  they  are 
wholly  unable  to  impart  it  to  their  pupils.  Your  system  obviates  this  difficulty, 
by  giving  the  teacher  a  resource  from  which  to  supply  his  own  deficiency,  and 
hints  for  the  successful  application  of  whatever  knowledge  of  the  art  he  may 
possess. 

Your  charts  are  in  use  in  the  Union  and  Select  schools  of  this  village,  and  also 
in  other  schools  of  this  town  and  county,  and  with  the  happiest  success.  The 
scholars,  charmed  with  the  novelty  which  the  system  continually  presents,  and 
the  ease  with  which  they  master  its  principles,  vie  with  each  other  in  their  eflforts 
to  excel,  and  are  rewarded  by  acquiring  a  beautiful  "  hand"  and  neat  mechanical 
execution— and  at  the  same  time,  the  teacher  is  relieved  from  the  perplexing 
practice  of  random  teaching,  so  universal  in  our  schools. 

JAMES  M.  PHINNEY, 
County  Superintendent,  Monroe  Co.,  East  Dist. 

Penfield,  Nov.  1,  1847.  » 

From  the  Rochester  Monthly  Educator. 
We  believe  that  Mr.  Fulton  is  the  first  author  who  has  attempted  to  teach  the 
art  of  penmanship  by  rule.  Heretofore,  imitation  has  been  almost  the  sole  princi- 
ple used  to  direct  the  student  in  acquiring  a  knowledge  of  chirography,  and  as 
every  teacher  has  a  system  peculiar  to  himself,  no  uniform  plan  of  instruction 
could  be  successfully  introduced  into  our  common  schools.  Mr.  Fulton  has  ren- 
dered an  essential  service  to  the  cause  of  education,  in  perfecting  a  system  which 
does  not,  at  every  change  of  teacher,  require  a  variation  in  the  handwriting  oi 
the  pupil.  One  advantage  which  must  result  to  the  teacher  from  the  use  of  these 
charts,  is  the  great  amount  of  time  and  labor  that  will  be  saved  thereby— the  old 
method  of  writing  separate  copies  for  each  scholar  being  entirely  dispensed  with 
We  feel  confident  that  teachers  and  parents  who  will  take  time  to  examine  this 
system  of  penmanship  must  be  convinced  of  its  superiority  over  all  others. 


Fulton  &  Eastman  s  Book-KeeiAng, 


FULTON  8l  EASTMAN'S  BOOK-KEEPING. 

A  PRACTICAL  SYSTEM  OF  BOOK-KEEPING  BY  SINGLE  EN  FRY 
Containing  three  distinct  forms  of  books,  adcipted  for  tho 
Farmer,  Mechanic,  and  Merchant — to  which  is  added  a  va- 
riety of  useful  forms  for  practical  use,  viz.:  Notes,  liills. 
Drafts,  Receipts,  (fee.  &c. :  also  a  Compendium  of  Rules  oi 
Evidence  appUcable  to  Books  of  Account,  and  of  Law  in 
reference  to  the  Collection  of  Promissory  Notes,  &c.  By 
Levi  S.  Fulton  and  G.  W.  Eastman,  authors  of  a  complete 
System  of  Penmanship. 

Rochester,  Feb.  J2,  1848. 

L.  S.  Fulton,  Esq. : 

Dear  Sir  .—I  have  examined  with  much  satisfaction  your  System  of  Book- Keep- 
ing, and  take  pleasure  in  recommending  its  adoption  to  my  immediate  friends 
and  others. 

It  is  simple  and  easily  reduced  to  practice,  and  possesses  a  peculiar  adaptation 
to  the  wants  of  the  community  for  which  you  design  it. 

The  plan  for  Merchants'  Books,  which  I  examined  more  critically  than  other 
portions  of  the  work,  is  very  neat,  compact,  and  economical,  and  must  ensure  d 
great  degree  of  accuracy  in  keeping  accounts. 
I  believe  your  work  will  meet  the  present  wants  of  community. 

Very  respectfully,  your  friend,  ELIJAH  BOTTUM, 

Book-keeper  for  John  M.  French  &  Co., 
Rochester,  N.  Y. 


I  have  examined  Messrs.  Fulton  &  Eastman's  "  Practical  System  of  Book- 
Keeping  by  Single  Entry,"  and  am  pleased  with  the  work.  As  a  branch  of  Edu- 
cation, Book-Keeping  is  well  deserving  a  high  estimation  ;  and,  I  will  add,  there 
is  none  of  equal  importance  and  utility  more  generally  neglected,  particularly  in 
our  public  schools. 

The  work  above  alluded  to,  is  plain,  simple,  and  comprehensive,  and  weTl 
adapted  to  meet  the  wants  of  the  business  community.  In  many  respects  I  deem 
It  superior  to  any  other  work  of  the  kind  with  which  I  am  acquainted.  I  shall 
recommend  it  to  the  schools  under  my  charge. 

JOHN  T.  MACKENZIE, 
Lyons,  May  8, 1848.  Town  Superintendent. 

Fulton  &  Eastman's  Book-Keeping.— We  had  supposed  that,  in  the  multi- 
plicity of  works  on  Book-Keeping,  hardly  any  thing  valuable  remained  to  be 
suggested  by  later  authors,  should  any  such  present  themselves.  But  we  have 
been  convinced  of  our  short-  sightedness  in  examining  the  work  with  the  above 
title,  now  before  us.  The  work  is  principally  designed  for  schools— for  common 
schools— but  should  be  in  the  hands  of  every  Farmer,  Mechanic,  and  Merchant 
in  the  land.  It  opens  with  a  system  of  account-keeping  for  farmers,  followed  by 
one  for  mechanics,  and  this,  in  turn,  by  an  admirable  and  comprehensive  system 
of  mercantile  Book-keeping,  which,  for  its  simplicity,  and  time  and  labor  savir 

'28 


Fulton  ct  Eastman  s  Book- Keeping. 


properties,  possesses  advantages  over  all  other  systems  with  which  we  are  ac- 
quainted. These  advantages  are  thus  set  forth  by  the  authors  in  their  preface, 
and  an  examination  of  the  work  will  convince  any  man  competent  to  judge,  that 
they  are  not  over-estimated  : 

"It  [the  sj'stem  spoken  of]  saves  more  than  one-third  of  the  time  in  journal- 
izing, and  at  least  three-fourths  of  the  labor  in  posting.  It  requires  but  twelve- 
lines  in  the  Ledger  to  post  a  year's  business,  while  in  the  ordinary  way  as  many 
pages  may  be  necessary,.  In  settling  with  a  person  at  the  end  of  a  year,  you  have 
only  to  refer  back  to  twelve  places  in  the  Journal  to  show  him  all  the  items  of 
his  account,  whereas  in  the  ordinary  manner  of  keeping  books  you  might  have  to 
refer  to  five  hundred." 

Part  II.  of  the  work,  which  was  prepared  by  a  distinguished  member  of  the 
bar,  comprises  "  rules  of  evidence  and  general  rules  of  law  in  relation  to  bills  of 
exchange,  promissory  and  chattel  notes,  checks,  books  of  account,  &c.,  together 
with  a  large  number  of  forms  useful  to  all  classes  of  business  men  ;  such  as  deeds, 
bonds,  mortgages,  bills  of  sale,  powers  of  attorney,  bills  of  exchange,  notes,  re- 
ceipts, &c. 

This  invaluable  work  contains  232  pages  of  duodecimo,  is  printed  in  the  best 
style  of  Messrs.  A.  S.  Barnes  &.  Co.,  of  New  York,  on  an  excellent  qu:ality  of  pa- 
per, and  is  afforded  at  the  very  low  price  of  50  cents  per  copy. — Wayne  Co.  Whig. 

Lyons,  May  8,  1848. 
I  have  examined  "Fulton  &  Eastman  s  Book-Keeping,"  and  regard  it  as  a  use- 
ful work  on  the  subject  of  which  it  mainly  treats.  Its  methodical  arrangement, 
its  simple  and  ready  modes  of  keeping  accounts,  adapted  to  the  business  of  the 
Farmer,  Mechanic,  or  Merchant  respectively,  and  the  neat  style  in  which  it  is 
executed,  recommend  very  strongly  its  use  in  primary  institutions  of  learning, 
and  especially  in  common  schools.  It  is  to  be  hoped  that  its  general  introduction 
as  a  school-book,  will  cause  the  art  of  Book-keeping  to  be  regarded  as  one  of  the 
indispensable  requisites  of  what  is  termed  a  good  English  education. 

JAMES  C.  SMITH. 

From  the  Albany  Spectator. 
Fulton  &  Eastman's  Book-Keeping. — New  York  :  A.  S.  Barnes  6c  Co., 
1848. 

We  are  very  much  pleased  with  the  design  and  execution  of  this  work.  It  is 
exceedingly  practical ;  being  by  single  entry,  containing  three  different  forms  ot 
books,  for  the  Farmer,  the  Merchant,  and  Mechanic.  To  these  are  added  notes, 
bills,  drafts,  receipts,  and  a  compendium  of  rules  of  evidence  applicable  to  books 
of  account,  and  of  law  in  reference  to  the  collection  of  promissory  notes.  A 
work  of  such  a  character,  and  of  so  much  practical  value,  speaks  for  itself,  and 
stands  in  need  of  no  commendation  from  us  to  ensure  it  a  large  ^ale  among  all 
classes.  . 

29 


Clark's  English  Grammar, 


SCIENCE  OF  THE   ENGLISH  LANGUAGE. 


CLARK'S  NEW  ENGLISH  GRAMMAR. 
A  Practical  Grammar,  in  which  Words,  Phrases,  and  Sentences 
a,re  classified,  according  to  their  offices,  and  their  relation  to 
each  other  :  illustrated  by  a  complete  system  of  Diagrams.  By 
S.  W.  Clark,  A.  M.     Price  50  cts. 

From  the  Rahway  Register. 
It  is  a  most  capital  work,  and  well  calculated,  if  we  mistake  not,  to  supersede, 
even  in  our  best  schools,  works  of  much  loftier  pretension.  The  peculiarity  of  its 
method  grew  out  of  the  best  practice  of  its  author  (as  he  himself  assures  us  in  its 
preface)  while  engaged  in  communicating  the  science  to  an  adult  class  ;  and  his 
success  was  fully  commensurate  with  the  happy  and  philosophic  design  he  has 
unfolded.  Technicality,  as  technicality,  our  author  unceremoniously  discards,  and 
substitutes  on  the  pupil's  part  rational  •practice  in  ascertaining  the  office  of  words  in 
sentences,  rather  than  the  usual  mode  of  perplexing  his  memory  with  their  mere 
names  and /orms. 

From  the  New  York  Tribune, 
"  The  Science  of  the  English  Language— A  Practical  Grammar,  in  which 
Words,  Phrases,  and  Sentences  are  classified  according  to  their  offices  and  their 
relation  to  each  other.  Illustrated  by  a  complete  system  of  Diagrams.  By  S.  W. 
Clark,  A.  M.,"  is  a  new  work  which  strikes  us  very  favorably.  Its  deviations  from 
older  books  of  the  kind  are  generally  judicious  and  often  important.  We  wish 
teachers  would  examine  it. 

From  the  Courier  and  Enquirer. 
"  A  Practical  Grammar  of  the  English  Language"  by  S.  W.  Clark,  A.  M.,  has 
just  been  published  by  Barnes  «fe  Co.  It  is  prepared  upon  a  new  plan,  to  meet  diffi- 
culties which  the  author  has  encountered  in  practical  instruction.  Grammar  and 
the  structure  of  language  are  taught  throughout  by  analysis,  and  in  a  way  which 
renders  their  acquisition  easy  and  satisfactory.  From  the  slight  examination, 
which  is  all  we  have  been  able  to  give  it,  we  are  convinced  it  has  points  of  very 
decided  superiority  over  any  of  the  elementary  works  in  common  use.  We  com- 
mend it  to  the  attention  of  all  who  are  engaged  in  instruction 

From  A.  R.  Simmons^  Ex- Superintendent  of  Bristol. 

Mr.  Clark: 

Dear  Sir  .-—From  a  thorough  examination  of  your  method  of  teaching  the  Eng- 
lish language,  I  am  prepared  to  give  it  my  unqualified  approbation.  It  is  a  plan 
original  and  beautiful— well  adapted  to  the  capacities  of  learners  of  every  age  and 
stage  of  advancement.  Believing  that  the  introduction  into  our  Common  Schools 
and  Academies  of  a  text-book  on  grammar  containing  your  system  and  method 
will  greatly  facilitate  the  acquisition  of  the  science  of  the  English  language,  I  re 
spectfully  suggest  that  it  be  permitted  to  come  before  the  public. 
Respectfully  yours, 

A.  R.  SIMMONIS,  Grammar  Teachei 

Bristol,  August  38,  184T 

30 


Clark's  English  Grammar. 


From  the  Geneva  Courier. 
Mr.  Clark^s  Grammar  is  a  work  of  merit  and  originality.  It  contains  an  etymo- 
logical chart  by  which  the  mode,  tense,  &c.,  of  a  verb,  or  the  gender,  person,  fee, 
of  a  noun,  or  the  different  fo^ms  of  any  part  of  speech,  can  be  determined  at  a 
erlance.  It  also  embraces  a  system  of  Diagrams,  which  illustrate  very  simply 
and  satisfactorily  the  relation  which  the  different  words  of  a  sentence  bear 
each  other.  The  student  of  grammar  must  be  greatly  assisted  by  the  introduction 
of  these  helps,  which  fumis)i  grammar  to  the  eye  as  well  as  to  the  mind. 

From  the  Geneva  Gazette. 
This  work  is  the  production  of  a  successful  teacher  in  our  o^^ti  county,  and 
has  grown  out  of  the  necessities  which  have  appeared  to  the  writer  to  exist,  in 
order  to  present  the  science  of  Grammar  in  a  proper  manner  to  the  attention  of 
the  scholar.  The  work  has  been' prepared  for  publication  by  the  author  at  the 
solicitation  of  teachers  of  high  character.  The  design  is,  in  many  respects,  ori- 
ginal, but  appears  to  be  based  on  sound  philosophical  principles  ;  and  the  work 
is  most  certainly  worthy  of  the  close  attention  and  examination  of  teachers. 

From  the  Ontario  Messenger. 
In  mechanical  execution,  the  book  is  a  good  one  ;  and  if  we  may  hazard  an 
opinion,  we  should  say  the  method  the  author  has  adapted  for  teaching  grammar, 
is  in  advance  of  any  thing  of  the  kind  we  have  ever  seen.  His  plan  of  using  Dia- 
grams in  explaining  the  structure  of  sentences,  is  a  feature  in  this  work,  which, 
among  many  others,  strikes  us  favorably,  and  which,  we  believe,  is  calculated  to 
present  at  one  glance,  what  many  pages  of  written  matter  in  the  grammars  now 
in  use  do  not  contain  in  an  intelligible  form.  Geometry  can  be  taught  without 
figures,  and  geography  without  pictures  or  maps  ;  but  no  one  in  our  day  would 
think  of  learning  either  of  these  sciences  without  the  aid  of  figurative  representa- 
tions ;  and  we  see  no  good  reason  why  this  "  system  of  diagrams"  is  not  equally 
useful  in  the  study  of  grammar.  The  brevity,  perspicuity,  and  comprehensive- 
ness of  this  work  are  certainly  rare  merits,  and  alone  would  commend  it  to  the 
favorable  consideration  of  teachers  and  learners.  Take  it  altogether,  we  think 
it  a  work  in  accordance  with  the  spirit  of  the  age,  and  we  wish  the  author  success 
in  his  labors  of  improvement. 

From  the  Seneca  Observer. 

It  is,  in  our  opinion,  a  valuable  work ;  the  best  calculated  of  any  which  has 
fallen  under  our  notice  to  impart  interest  to  a  study  not  usually  very  attractive. 
We  commend  tliis  work  to  the  notice  of  our  teachers  :  we  are  confident  it  will 
be  favorably  received  by  them. 


Clark's  Grammar  I  have  never  seen  equalled  for  practicability^  which  is  of  the 
utmost  importance  in  all  school-books. 

S.  B.  CLARK, 

January,  1848.  Principal  of  Scarborough  Academy,  Maine 

The  Grammar  is  just  such  a  book  as  I  wanted,  and  I  shall  make  it  the  text-book 
in  my  school. 

WILLIAM  BRICKLEY, 
February,  1S48.  Teacher,  of  Canastota,  N.  Y 

31 


Clark's  English  Grammar, 


From  Professor  Brittan,  Principal  of  the  Lyons  Union  School. 
Messrs.  A.  S.  Barnes  &  Co.  : 

I  have,  uiuler  my  immediate  instruction  in  English  Grammar,  a  class  of  more 
ihan  fifty  ladies  and  gentlemen  from  the  Teachers'  Department,  who,  having 
studied  tiie  grammars  in  common  use,  concur  with  me  in  expressing  a  decided 
preference  for  "  Clark's  New  Grammar,"  which  we  have  used  as  a  text-book 
since  its  publication,  and  which  will  be  retained  as  such  in  this  school  hereafter. 

The  distinguishing  peculiarities  of  the  work  are  two  ;  and  in  these  much  of  its 
merit  consists.  The  first,  is  the  logical  examination  of  a  sentence  as  the  first  step 
m  the  study  of  language,  or  grammar.  By  this  process  the  pupil  readily  per- 
ceives that  words  are  the  instruments  which  the  mind  employs  to  perfect  and  to 
express  its  own  conceptions  ;  that  the  principal  words  in  a  sentence  may  be  so 
modified  in  their  significations  by  other  words  and  by  phrases,  as  to  express  the 
exact  proposition  or  train  of  thought  designed  to  be  communicated;  and  that 
words,  phrases,  and  sentences  may  be  most  properly  distinguished  and  classified 
according  to  the  office  they  perform. 

The  other  distinguishing  peculiarity  of  the  work  is  a  system  of  Diagrams  ;  and 
a  most  happy  expedient  it  is  to  unfold  to  the  eye  the  mutual  relation  and  depend- 
ence of  words  and  sentences,  as  used  for  the  purpose  of  delineating  thought. 

I  believe  it  only  requires  a  careful  examination  by  teachers,  and  those  who 
have  the  supervision  of  our  educational  interests,  to  secure  for  this  work  a  speedy 
introduction  into  all  our  schools.  Yours  very  truly, 

N.  BRITTAN. 

Lyons  Union  School,  February  21,  1848. 


From  H.  G.  Winsloiv,  A.  M.,  Principal  of  Mount  Morris  Union  School. 
I  have  examined  your  work  on  Grammar,  and  do  not  hesitate  to  pronounce  it 
superior  to  any  work  with  which  I  am  acquainted.   I  shall  introduce  it  into  the 
Mount  Morris  Union  School  at  the  first  proper  opportunity. 

Yours  truly,  H.  G.  WINSLOW. 

From  S.  N.  Sweet,  Esq.,  Counsellor  at  Law. 

Professor  Clark's  new  work  on  Grammar,  containing  Diagrams  illustrative  of  his 
system,  is,  in  my  opinion,  a  most  excellent  treatise  on  "  the  Science  of  the  Eng- 
lish Language."  The  author  has  studiously  and  properly  excluded  from  his  book 
the  technicalities,  jargon,  and  ambiguity  which  so  often  render  attempts  to  teach 
grammar  unpleasant,  if  not  impracticable.    *   *  * 

The  inductive  plan  which  he  iias  adopted,  and  of  which  he  is,  in  teaching  gram 
mar,  the  originator,  is  admirably  adapted  to  the  great  purposes  of  both  teaching 
and  learning  the  important  science  of  our  language. 

SAMUEL  N.  SWEET,  Author  of  "  Sweet's  Elocution." 

Whitesborough,  January  10,  1848. 

From  H.  O'Dell,  Esq.,  Teacher  and  Ex- Superintendent  of  Hopewell. 
S.  W.  Clark  : 

Sir ;— I  have  examined  your  Grammar,  and  have  no  hesitation  m  recommend- 
ing it  to  those  engaged  in  teaching  the  youth  of  our  country  as  the  work  on  the 
subject  of  grammar  which  the  present  age  of  improvement  demands.  I  have  in- 
troduced it  into  my  school,  and  find  it  admirably  adapted  to  wake  up  the  minds  of 
the  students  of  grammar,  especially  the  younger  portion. 

Yours,  H.  O'DELL. 

32 


Valuahle  Music  Books. 


VALUABLE    MUSIC  BOOKS. 

Edited  by  Geo.  Kingsley,  Professor  of  Music— author  of  *'  Social 
Choir,"    Sacred  Choir,"  &c. 


KINGSLEY'S  JUVENILE  CHOIR 
A  selection  of  the  choicest  melodies  from  the  German,  Italian,  French,  Eng- 
lish, and  American  composers.   Designed  for  public  and  private  schools,  and  for 
young  classes  in  academies  and  seminaries.     Price  40  cts. 

KIXGSLEY'S  YOUNG  LADIES'  HARP. 
A  selection  of  secular  and  sacred  music,  arranged  in  two  and  three  parts,  with 
a  Piano  Accompaniment.    Designed  for  female  seminaries,  and  the  social 
circle.   Price  75  cts. 

KINGSLEY'S  HARP  OF  DAVID. 

A  collection  of  Church  Music,  consisting  of  selections  from  the  most  dis- 
tinguished composers,  together  with  original  pieces  by  the  editor— also  a  pro- 
gressive system  of  Elementary  instruction  for  pupils.    Price  $1.00. 


Extract  of  a  Letter  from  Mr.  Gilbert  Combs,  Principal  of  the  Female  Seminary  f 

Philadelphia. 

Among  the  numerous  works  now  prepared  for  youth,  few  are  worthy  of  taking 
a  higher  rank  than  Kingsley's  Juvenile  Choir.  Arranged  in  a  style  calculated  to 
enlist  the  youthful  feelings,  it  is  still  free  from  common-place  or  imperfect  har- 
monies. It  is  chaste  in  style,  simple  and  pure  in  sentiment,  and  vigorous  in 
tone.  Much  of  the  music  is  original,  and  the  favorite  airs  that  are  copied,  are 
much  improved  in  harmony  and  adaptation.  *******  a  judicious  teacher, 
with  the  aid  of  such  a  manual,  can  hardly  fail  to  produce  good  scholars.  *  *  * 

From  the  Louisville  Journal. 
KixGSLET's  Juvenile  Choir.— This  is  the  title  of  a  delightful  little  collection 
of  vocal  music  for  the  use  of  cMldren.  The  tunes  are  well  selected,  and  there  is 
a  ^eat  deal  of  beautiful  poetry  which  children  will  fed.  It  would  be  a  treat  to 
hear  some  of  these  songs  sung  by  a  school  of  sweet  young  voices.  Music 
should  be  taught  in  every  primary  school.  Nothing  can  be  better  for  cultivating 
the  taste  and  sweetening  the  affections.  Men  are  too  much  inclined  to  believe 
that  the  only  object  in  education  is  to  enable  the  pupil  to  count  up  dollars  and 
cents.  They  forget  that  there  are  other  objects  much  more  important  than  to 
make  of  man  a  good  calculating  machine. 

From  the  Milicaukie  Sentinel. 
Kingsley's  Habp  of  David.— This  is  an  excellent  collection  of  Church 
Music,  consisting  of  the  best  selections  from  distinguished  composers,  and  a 
number  of  original  pieces.  It  is  compHed  by  George  Kuigsley,  professor  of 
music  and  author  of  "  The  Sacred  Choir,"  &c.  By  way  of  preface,  there  is  a  very 
complete  and  intelligible  elementary  course  of  instruction  in  vocal  music.  The  work 
is  very  neatly  printed  and  got  up,  throughout,  in  exceUent  taste.  It  is  not  only  a 
useful  assistant  for  the  choirs  of  churches,  but  may  be  introduced  with  advantage 
into  the  school-room  and  the  family  circle.  It  embraces  360  pages,  and  contains 
no  less  than  317  different  tunes.  We  believe,  indeed,  that  it  is  the  most  com- 
plete collection  of  Church  Music  now  in  print. 

33 


Valuable  Music  Books. 


From  the  Nao  York  Courier  Jf  Enquirer. 
The  Youno  Ladies'  Harp.  By  George  Kingsley.— This  is  a  very  judicioub 
Belection  and  arrangement  from  the  works  of  the  most  popular  Italian  com- 
posers ;  the  music  selected,  being  adapted  to  English  words  frequently  selected 
from  the  best  of  our  American  poets.  Some  of  the  simplest  German  and  of  the 
best  English  vocal  compositions  or  arrangements  from  mstrumental  works,  will 
be  found  scattered  th-ough  the  volume.  It  is  a  publication  admirably  calculated 
to  beget  and  diffuse  a  taste  for  the  better  sort  of  music.  It  is  distinctly  printed  on 
white  paper. 

Extract  from  the  New  York  Evangelist. 
The  best  pieces,  besides  the  old  standards  w^hich  make  their  appearance  m 
every  book,  are  decidedly  those  of  Mr.  Kingsley's  own  composition— the  grace 
and  sentiment  of  whose  style,  the  public  know  and  appreciate. 

From  the  Christian  Observer. 
Young  Ladies'  Harp.— This  is  a  beautiful  book  of  music,  printed  in  superior 
style,  on  fair  paper,  with  an  embellished  title-page— containing  a  choice  collec- 
tion of  pieces,  which  will  render  it  a  favorite  in  those  parlors  where  music  is 
cultivated.  The  compositions  in  the  secular  department  are  chaste  and  elevated 
in  spirit  and  character.  The  sacred  pieces  comprise  an  excellent  selection,  em- 
bracing among  others  the  following—"  Bow  down  thine  ear,  O  Lord  ;"  Brattle- 
Street ;  China;  Emanuel ;  Greenwood ;  Haydn;  Messiah;  Northampton  ;  Pren- 
tiss ;  Siloam,  &c.,  &c. 

From  "  Wrighfs  Paper"  for  the  Dissemination  of  Useful  Knowledge. 

The  Juvenile  Choir.- A  selection  of  the  choicest  melodies  from  the  German, 
French,  English,  and  American  composers— designed  for  public  and  private 
schools,  and  for  young  classes  in  academies  and  seminaries— is  the  first  of  a  series 
of  musical  works  prepared  by  Prof.  Kingsley,  and  which  appears  to  us  admirably 
adapted  to  the  use  of  the  young  in  public  and  private  schools.  There  is  a  purity 
of  sentiment  in  the  selections  made  by  Prof.  Kingsley  for  his  series  which  is  com- 
mendable. We  recommend  this  series  to  the  attention  of  parents  and  teachers 
opposed  to  the  low,  debasing,  doggerel  rhymes  so  widely  disseminated. 

The  second  in  this  series  of  musical  works  is  styled  "  The  Young  Ladies' 
Harp  ;"  a  selection  of  secular  and  sacred  music  from  distinguished  composers- 
arranged  m  two  and  three  parts,  with  a  Piano  Accompaniment,  designed  for 
female  academies,  seminaries,  and  senior  classes  in  public  and  private  schools, 
and  for  the  social  circle. 

The  third  is  entitled,  "  The  Har?*  cf  David."— A  collection  of  Church  Music, 
consisting  of  selections  from  the  most  distinguished  composers,  and  a  number  of 
original  pieces  by  the  editor — with  a  progressive  system  of  elementary  instruc- 
tion for  pupils.  This  is  the  last  of  the  series,  and  we  can  truly  say  of  this,  as 
also  of  the  other  two,  they  are  excellent,  and  well  worthy  the  extensive  patron- 
age they  have  received. 

From  the  Connecticut  Whig. 
The  Juvenile  Choir.— This  is  the  best  musical  manual  for  children  we  ever 
saw.    George  Kingsley,  its  author,  is  a  composer  of  exquisite  taste,  and  some  of 
his  collections  have  never  been  surpassed.    This  book  has  been  introduced  oa 
account  of  its  intrinsic  excellence,  into  the  Hartford  Female  wSprainary. 


34 


Mansfield's  Life  of  General  Scott. 


MANSFIELD'S  LIFE  OF  GENERAL  SCOTT. 


THE  LIFE  OF  GENERAL  WINFIELD  SCOTT, 

BY  EDWARD  D.  MANSFIELD. 

This  work  gives  a  full  and  faithful  narrative  of  the  important  events 
with  which  the  name  and  ser^dces  of  General  Scott  have  been  con- 
nected. It  contains  numerous  and  ample  references  to  all  the  sources 
and  documents  from  which  the  facts  of  the  history  are  drawn.  Illus- 
trated with  Maps  and  Engravings.    12mo.  350  pages. 

From  the  New  York  Tribune. 
We  have  looked  through  it  sufficiently  to  say  with  confidence  that  it  is  well 
done— a  valuable  addition  to  the  best  of  American  biographies.  Mr.  Mansfield 
does  his  work  thoroughly,  yet  is  careful  not  to  overdo  it,  so  that  his  Life  is  some- 
thing better  than  the  fulsome  panegyrics  of  which  this  class  of  works  is  too  gen- 
erally composed.  General  Scott  has  been  connected  with  some  of  the  most 
stirrmg  events  in  our  national  history,  and  the  simple  recital  of  his  daring  deeds 
warms  the  blood  like  wine.  We  commend  this  well  prmted  volume  to  general 
perusal. 

From  the  N.  Y.  Courier  and  Enquirer. 

This  volume  may,  both  from  its  design  and  its  execution,  be  classed  among 
what  the  French  anpropriately  call  "  memoirs,  to  serve  the  cause  of  history," 
blending,  as  it  necessarily  does,  with  all  the  attraction  of  biographical  incidents, 
much  of  the  leading  events  of  the  time.  It  is  also  a  contribution  to  the  fund  of  " 
true  national  glory,  that  which  is  made  up  of  the  self-sacrificing,  meritorious,  and 
perilous  services,  m  whatever  career,  of  the  devoted  sons  of  the  nation. 

From  the  U.  S.  Gazette,  {Philadelphia.) 
A  beautiful  octavo  volume,  by  a  gentleman  of  Cinciimati,  contains  the  above 
welcome  history.  Among  the  many  biographies  of  the  eminent  officers  of  the 
army,  we  have  found  that  that  of  General  Scott  did  not  occupy  its  proper  place  ; 
but  in  the  "  authentic  and  unimpeachable  history"  of  his  eventful  life  now  pre- 
sented, that  w^ant  is  satisfied. 

From  the  Cleveland  {Ohio)  Daily  Herald. 
We  are  always  rejoiced  to  see  a  new  book  about  America,  and  our  country 
men,  by  an  American— especially  when  that  book  relates  to  our  history  as  a  na- 
tion,' or  unrolls  those  stirring  events  in  which  our  prominent  men.  both  dead  and 
living,  have  been  actors.  As  such  we  hail  with  peculiar  delight  and  pride  the 
work  now  before  us  ;  it  has  been  written  by  an  American  hand,  and  dictated  by 
an  American  heart— a  heart  deeply  imbued  with  a  love  of  his  native  land,  its 
institutions,  and  distinguished  men. 


History  of  the  Mexican  W xr. 


THE   MEXICAN  WAR: 

A  History  of  its  Origin,  with  a  detailed  Account  of  the  Victories 
which  terminated  in  the  surrender  of  the  Capital,  with  the  Official 
Despatches  of  the  Generals.  By  Edward  D.  Mansfield,  Esq 
Illustrated  with  numerous  Engravings. 

From  the  Philadelphia  North  American. 
Mr.  Mansfield  is  a  writer  of  superior  merit.  His  style  is  clear,  nervous,  and 
impressive,  and,  while  he  does  not  encumber  his  narrative  with  useless  ornament, 
his  illustrations  are  singularly  apt  and  striking.  A  graduate  of  West  Point,  he  is 
of  course  familiar  with  military  operations  ;  a  close  and  well  read  student,  he  has 
omitted  no  sources  of  information  necessary  to  the  purposes  of  his  work  ;  and  a 
shrewd  and  investigating  observer,  he  sees  in  events  not  alone  their  outward  as- 
pects but  the  germs  which  they  contain  of  future  development.  Thus  qualified, 
it  need  hardly  be  said  that  his  history  of  the  war  with  Mexico  deserves  the  am- 
plest commendation. 

From  the  New  York  Tribune. 
A  clear,  comprehensive,  and  manly  history  of  the  war,  is  needed ;  and  we  are 
glad  to  find  this  desideratum  supplied  by  Mr.  Mansfield's  work. 


From  the  New  York  Courier  and  Enquirer. 

This  is  really  a  history,  and  not  an  adventurer's  pamphlet  destined  to  live  for 
the  hour  and  then  be  forgotten.  It  is  a  volume  of  some  320  pages,  carefully  writ- 
ten, from  authorities  weighed  and  collated  by  an  experienced  writer,  educated 
at  West  Point,  and  therefore  imbued  with  a  just  spirit  and  sound  views,  illustra- 
ted by  plans  of  the  battles,  and  authenticated  by  the  chief  oflacial  despatches. 

The  whole  campaign  on  the  Rio  Grande,  and  that,  unequalled  m  brilhancy  m 
any  annals,  from  Vera  Cruz  to  the  city  of  Mexico,  are  unrolled  before  the  eyes 
of  the  reader,  and  he  follows  through  the  spirited  pages  of  the  narrative,  the  dar- 
ing bands  so  inferior-m  every  thing  but  indomitable  will  and  unwavering  self-re- 
liance, and  military  skill  and  arms-to  the  hosts  that  opposed  them,  but  opposed 
in  vain. 

We  commend  this  book  cordially  to  our  readers. 


From  the  Baptist  Register,  Utica. 
The  military  studies  of  the  talented  editor  of  the  Cincinnati  Chronicle,  admi- 
rably qualified  him  to  give  a  truthful  history  of  the  stirring  events  connected  with 
the  unhappy  war  now  ragmg  with  a  sister  republic  ;  and  though  he  declares  m 
his  preface  that  he  felt  no  pleasure  in  tracing  the  causes,  or  in  contemplating  the 
Drogress  and  final  consequences  of  the  conflict,  yet  his  graphic  pages  give  proof 
of  his  ability  and  disposition  to  do  justice  to  the  important  portion  of  our  nation  s 
history  he  has  recorded.  The  very  respectable  house  publishing  the  book,  have 
done  great  credit  to  the  author  and  his  work,  as  well  as  to  themselves,  in  the 
handsome  style  in  which  they  have  sent  it  forth. 

(aa) 


Chamhers'  Educational  Course. 


CHAMBERS'  EDUCATIONAL  COURSE. 

NEW  AMERICAN  EDITIONS, 

FROM  THE  REVISED  AND  IMPROVED  EDINBURGH  EDITION. 

BY  D.  M.  REESE,  M.D.,  LL.D. 


These  Works,  published  under  the  direction  of  the  Messrs.  Chambers,  of 
Edinburgh,  who  are  known  in  Great  Britain  and  America  by  their  numerous 
and  valuable  publications,  are  intended  especially  for  schools,  and  for  the  diffusion 
of  intelligence  on  all  scientific  and  practical  subjects.  They  have  secured  for 
the  authorship  of  this  series  the  labors  of  some  of  the  first  Professors  in  Scotland 
in  the  several  branches.  The  different  volumes  are  numerously  illustrated,  and 
will  be  found  to  be  admirably  adapted,  as  Text-Books,  and  also  as  Library 
Books,  for  the  i?chools  and  Famihes  of  the  United  States. 

L   TREASURY  OF  KNOWLEDGE. 

Three  Parts  in  One. 
Part  I.— Elementary  Lessoxs  in  Common  Things. 
Part  TI.— Practical  Lessons  on  Common  Objects. 
Part  IIL— Introduction  to  the  Sciences. 

n.    ELEMENTS  OF  DRAWING  AND  FERSPEOTIYB. 
Two  Parts  in  One. 
Part  I. — Embraces  Exercises  for  the  Slate  and  Blackboard. 
Part  II. — Embraces  Exercises  in  Perspectr^e,  Light  and  Shade 

ni.    ELEMENTS  OF  NATURAL  PHILOSOPHY. 

Three  Parts  in  One. 
Part  L — Laws  of  Matter  and  3Iotion. 
Part  IL — 3Iechantcs. 

Part  III.— Hydrostatics,  Hydraulics,  and  Pneumatics. 

IV.   ELEMENTS  OF  CHEMISTRY.    By  D.  B.  Reid,  M.D.,  F.R.S.E. 

With  Illustrations  of  the  Chemical  Phenomena  of  Daily  Life, 
and  a  Series  of  Practical  Experiments. 

V.   ELEMENTS  OF  GEOLOGY.   By  David  Page. 

VL    ELEMENTS  OF  PHYSIOLOGY. 

Two  Parts  in  One. 

Part  I. — Vegetable  Physiology.— Treats  of  the  general  Structure  and  Func- 
tions of  Plants — their  various  Organs,  and  the  Terms  by  which  they  are  disiin- 
guis^ied— their  Modes  of  Growth  and  Reproduction— their  Geographical  Distri- 
bution, and  extensive  Utility  in  the  Scheme  of  Creation. 

Part  H.— Animal  Physiology.— Treats  of  the  Organization,  Life,  and  Classi- 
fication of  Animals — their  Mastication — Deglutition — Digestion — Circulation  o. 
Blood — Pv.espiration— Secretion  and  Nutrition — Exhalation,  Absorption,  and  Lo- 
comotion— The  Bones  Muscles,  &c.— The  Nervous  System,  the  Senses,  and 
Reproduction. 


VII.   ELEMENTS  OF  ZOOLOGY. 


Theory  and  Practice  of  Teaching. 


THEORY  AND  PRACTICE  OF  TEACHING; 

OR, 

THE  MOTIVES  AND  METHODS 

OF 

GOOD  S€HOOL.-IL£EPI]¥0. 


BY  DAYID  P.  PAGE,  A.M. 

LATE  PRINCIPAL  OF  THE  STATE  NORMAL  SCHOOL,  ALBANY,  NEW  YORK. 


CONTENTS. 

The  Spirit  oj  the  Teacher  .—Responsibility  of  the  Teacher:  1.  The  Neglected  Tree, 

2.  Extent  of  Responsibility,  3.  The  Auburn  Prison. — Habits  of  the  Teacher.— 
Literary  Qualifications  of  the  Teacher. — Right  Views  of  Education. — Right  Modes  of 
Teaching:  1.  Pouring-in  Process,  2.  The  Drawing-out  Process,  3.  The  more  ex- 
cellent Way,  4.  Waking  up  Mind,  5.  Remarks. — Conducting  Recitations.— Ex 
citing  an  Interest  in  Study:  1.  Incentives.  ..Emulation,  2.  Prizes  and  Rewards, 

3.  Proper  Incentives.— ScAooZ  Government:  1.  Requisites  in  the  Teacher  for 
Government,  2.  Means  of  securing  Good  Order,  3.  Punishments  . . .  Improper  . . 
Proper,  4.  Corporal  Punishment,  5.  Limitations  and  Suggestions.— ScAooZ  Ar- 
rangements :  1.  Plan  of  Day's  Work,  2.  Interruptions,  3.  Recesses,  4.  Assignment 
of  Lessons,  5.  Reviews,  6.  Examinations  ...  Exhibitions  ...  Celebrations —TAe 
Teacher's  Relation  to  the  Parents  of  his  Pupils.— The  Teacher's  Care  of  his  Health.— 
The  Teacher's  Relation  to  his  Profession. — Miscellaneous  Suggestions  :  L  Things  to 
be  avoided,  2.  Things  to  be  performed. — The  Rewards  of  the  Teacher. 


This  work  has  had  its  origin  in  a  desire  to  contribute  something  towards  ele- 
vating an  important  and  rising  profession.  Its  matter  comprises  the  substance  of  a 
part  of  the  course  of  lectures  addressed  to  the  classes  of  the  Institution  under  my 
charge,  during  the  past  two  years.  Those  lectures,  unwritten  at  first,  were  de- 
livered in  a  familiar,  colloquial  style,— their  main  object  being  the  Inculcation  of 
such  practical  views  as  would  best  promote  the  improvement  of  the  teacher.  In 
wriiing  the  matter  out  for  the  press,  the  same  style,  to  considerable  extent,  has 
been  retained,~as  I  have  written  with  an  aim  at  usefulness  rather  than  rhetorical 
elfect. 

If  the  term  theory  in  the  title  suggests  to  any  mind  the  bad  sense  sometimes-con- 
veyed by  that  word,  I  would  simply  say,  that  I  have  not  been  dealing  in  the 
speculative  dreams  of  the  closet,  but  in  convictions  derived  from  the  realities  of 
ihe  schoolroom  during  some  twenty  years  of  actual  service  as  a  teacher.  Theory 
ffiay  justly  mean  the  science  distinguished  from  the  art  of  Teaching,— but  as  in 
practice  these  should  never  be  divorced,  so  in  the  following  chapters  I  have  en- 
leavored  constantly  to  illustrate  the  one  by  the  ether. 


Barnard  on  School  Architecture^  dc. 


SCHOOL  ARCHITECTURE 

OR, 

CONTRIBUTIONS  TO  THE  IMPROVEMENT  OF  SCHOOL-HOUSES 

IN  THE 

UNITED  STATES. 

BY  HENRY  BARNARD, 

COMMISSIONER  OF  PUBLIC  SCHOOLS  IN  RHODE  ISLAND. 


CONTENTS. 

INTRODUCTION. 
Condition  of  School-houses  in  Massa- 
chusetts, New  York,  Vermont,  New- 
Hampshire,  Connecticut,  Maine,  Rh. 
Island,  Michigan. 

SCHOOL  ARCHITECTURE. 
I.  Common  Errors  to  be  avoided. 
II.  General  Principles    to    be  ob- 
served. 
m.  Plans  op  School-houses. 

1.  Plans  recommended  by  practi-^ 

cal  Teachers. 

2.  Plans  and  Description  of  School- 

houses  recently  erected. 


From  the  Vermont  Chronicle. 

Mr.  Barnard,  when  Secretary  of  the  Board  of  Commissioners  of  Common  Schools 
in  Connecticut,  devoted  much  time  to  School-houses,  lectured  upon  the  subject, 
and  published  his  views  in  the  School  Journal,  which  he  then  edited.  Since  that 
date,  (1841,)  his  Essay  has  been  repeatedly  published,  each  time  with  additional 
plans  and  descriptions.  Parts  of  it  have  been  extensively  copied,  and  its  influence 
has  been  felt  in  all  parts  of  the  country.  He  has  now  enlarged  it  to  a  handsome 
volume,  which  is  published  in  a  style  becoming  its  importance  and  excellence.  Nc 
other  writer  on  the  subject  is  to  be  compared  with  Mr.  Barnard  for  the  fulness  and 
variety  of  his  materials,  and  the  completeness  of  his  work  in  regard  to  all  ihe 
points  that  are  to  be  considered  in  the  building  and  furnishing  of  school-houses. 

Mr.  Barnard  does  not  confine  himself  to  any  one  plan,  but  exhibits  fully  a  great 
variety  of  excellent  models  for  buildings,  seats,  desks,  warming,  ventilating,  &c., 
&c.,  for  Primary  Schools,  High  Schools,  Normal  Schools;  with  numerous  cuts 
and  descriptions  of  buildings  and  parts  of  buildings  in  use  in  places  where  most  at- 
tention has  been  given  to  the  subject.  At  the  close  we  have  nearly  a  hundred  pages 
on  school  apparatus  and  library,  care  of  school-houses,  school-regulations,  books 
on  education,  suggestions  for  improvement,  &:c.  The  whole  book  is  replete  with 
information,  and  we  heartily  recommend  it  as  one  that  ought  to  be  accessible  in 
every  school-district.  No  school-house  should  be  built  or  altered  without  consult- 
ing it. 


3.  Plans  for  School-houses,  con- 
taining Apartments   for  the 
Teacher. 
IV.  Apparatus. 
V,  Library. 
VI.  Miscellaneous  Suggestions. 

1.  Plans  of  Ventilation  and  Warm- 
ing. 

2.  School  Furniture,  Fixtures,  fee. 

3.  Regulations  for  the  Use  and 
Preservation  of  School-houses, 
Furniture,  &c. 

4.  Dedicatory  Exercises. 

5.  Priced  Catalogue  of  Books  on 
Education,  Apparatus,  Maps, 
&c. 


